Hokay, rare as it is, sometimes Macs do start doing weird things from time to time, and rather than respond to every PM and help thread individually for simple things, I figured I'd make a "try this first" thread.

First, OS X is a rather good UNIX. It's actually the most widely used commercial UNIX in the world. Being UNIX, it will endeavor to tell you all sorts of useful things that are potentially wrong with itself, via system logs.

So, if something is acting weird, the very first thing you should do is check said logs. This is accomplished with the Console application found in Applications -> Utilities. Its default action is to display the system log, which is generally what you want.

Take note of anything weird and/or bad going on in this log. Weird and/or bad things can easily be identified by a preface of WARNING or ERROR. Most things in the system log are not cause for concern, but this log contains very useful debugging information which can help you identify problems.

Other logs to pay attention to are the Diagnostic Messages log-- again, most of this is filled up by processes doing entirely normal things, like your machine waking up, airport link coming up, and so on and so forth. However, when things are going wrong, it will also contain useful information that can help you diagnose the problem.

Now, assuming you cannot see any obvious error messages suggesting something odd is going on, check the following in this order of priority.

First, check your network. Network issues will cause all sorts of "slowness", which really isn't. It's daemons and applications trying to perform a function, then timing out because a given resource is not available. For example, if it takes a very long time to hit a web site, chances are good you have a DNS issue. Make certain your network settings from your ISP are kept current, and consider adding a few third-party DNS servers to your list. You can obtain a few close, third party DNS hosts from this site.

Second, run periodic. Not many people know about this, but like any UNIX, OS X has a number of scheduled tasks which generally run automatically in the (very) early morning. These tasks include things like rotating logs, removing temporary files, preening file systems, and so on and so forth. For servers, which run 24x7, these things always happen. For desktop machines that are often shut down, they do not. So, it is a good practice to run them manually every so often, unless you keep your machine running constantly. (Note: Plugged in and sleeping does not equal running)

So, to run periodic, do the following:

1) Open a terminal (Applications -> Utilities -> Terminal)

2) Run the following command:

sudo periodic daily weekly monthly

You will be prompted for your password, once entered, periodic will run its housekeeping tasks. There is no feedback from the terminal when running. Once the process is complete, the shell prompt will return. You can then exit terminal.

Third, repair disk permissions. Like any UNIX, OS X uses owner/group/other file permissions. Should these permissions get mis-set, potentially by software installers, delta updates, or what have you, weird things can occasionally happen. Note: If you discover system.log entries to the tune of a given file being unable to execute or be read by the system, that is an obvious indication of a file permission problem.

To repair permissions, open Disk Utility (Applications -> Utilities -> Disk Utility), click your system volume (which is the hard drive icon with the same name as the one on your desktop), and then click "Repair Disk Permissions". Note any RED errors that are reported, or any files that cannot be corrected. This is very useful diagnostic information to pass along should it be required.

Fourth, reboot the thing. While OS X can theoretically run forever without being rebooted, save for system updates that modify running processes or the kernel, it can occasionally benefit from a reboot. If you sleep/wake your machine a lot, or have any slowness or file space issues, a weekly reboot can solve some of these issues. One thing to take note of, if a reboot does solve what ails, is what resident programs you have running constantly. Any third party software, be it an IM application, dashboard widget, or what have you, may well have a memory leak or other issue which is causing your problem. And speaking of that...

Fifth, take note of any recently installed software. While most commonly available commercial software is put through a pretty rigorous QA process before being released, occasionally things are missed, even by Apple, which can cause system weirdness. If your problems have recently started, think back a few days and note any new software you have installed. It may be the cause of your problem. If temporarily shutting down or uninstalling the software fixes it, contact the vendor with the problem.

Sixth, reinstall the recent combo updater. Software Update periodically updates OS X via software deltas, meaning only the updates you do not currently have are downloaded and installed. Occasionally this process can pick up a glitch and break stuff. If your machine has started running slow or generally doing weird things, and you recently installed updates via Software Update, try the following:

1) Download the most recent Combo Updater for your version of OS X. The most recent updates can be found here. For example, this is the 10.6.2 Combo Updater for Snow Leopard. ALWAYS check for the most recent version, as by the time you read this, 10.6.2 may not be the latest Combo Updater.

2) Mount the DMG image and run the updater.

Lastly, try repairing your hard drive. Hard Drive issues are the primary cause of "slowness" and other weird behavior that is not attributed to software or OS problems. So, as a last resort, try the following:

1) Insert your OS X installation DVD and reboot. At the "bong" hold the "C" key to boot from the DVD.

2) Once the installation program starts, exit the installer and select Disk Utility from the Utilities option in the menu bar.

3) Select your system volume per the repairing disk permissions example above.

4) Click "repair disk". Note any RED errors and other bad information. Write them down for further diagnostic. Any SMART or drive hardware issues may mean your hard drive is failing, if so you should back up your data NOW, and have it replaced. If your machine is still under warranty, or you have AppleCare, this will be done free of charge.

If your machine, especially laptops, is making any sort of odd whining or clicking sound, this is almost certainly a hard drive issue.

If it still isn't working how it should, or you have discovered hard drive or other hardware issues, take your machine to your local Apple Store or VAR. Make a reservation first so you don't have to stand around like an idiot right here.

The Apple techs will be able to run further hardware diagnostics on your machine and tell you if anything else is wrong, be it a memory or logic board issue. It's rare, but it happens.

Alternatively, you can attempt an archive and reinstall of OS X, however, this is mostly a waste of time unless you are absolutely certain you do not have a hardware issues.

A few other things to note:

No, you don't have a virus. As common a knee-jerk reaction as it is among windows users, the chances of your Mac having any sort of malware or "virus" is next to none. About the only way such a thing could happen were if you were to download some "questionable" software package from some warez site or something, and then execute a specifically engineered program, and give it your admin password. If you do this, you are an idiot.

Barring that, however, it is virtually impossible for OS X to be remotely compromised by a worm or trojan or any of the plethora of crap that floats around affecting windows. Some people claim this is due to teenage russian worm authors "not caring" to attack OS X because it has too small a market share, or other such nonsense. People who suggest such, are idiots. The reason OS X can not be remotely compromised via any method that is remotely easy or even plausible is due to the fundamental design differences of the underpinning UNIX OS. It is extremely secure, having been designed as a multi-user network operating system, something windows is not. It does not have eleventy billion remote exploits because it is, quite simply, made better.