How to get your Mac playing nicely again. Quick fixes for 10 common Mac problems
Force quitting an app that is not responding - If an application is not responding, if it's continually bouncing in the Dock or appears to have frozen, you may need to force quit it. You can do this by holding Command+Alt+[esc] to bring up the Force Quit window, or by choosing Force Quit from the Apple menu. After quitting the program, try relaunching it. If it still doesn't work it may be incompatible with your version of OS X. Thankfully, force quitting programs on Macs nowadays doesn't generally affect the stability of the system and doesn't require a restart.
Making some space and adding more RAM - If your Mac is feeling sluggish and simple tasks are taking longer than usual to do, check that the boot drive isn't getting too full by selecting it and pressing Command+[I] to Get Info on it. Look at its size and how much available free space is left on it. If it's less than 20% of the drive's size that's free, you'll need to offload some stuff either onto DVDs or onto external hard drives to free up space. Good candidates include movies, iPhoto and iTunes libraries, plus any applications you don't use much. Also look at fitting more RAM to your Mac as this will also help greatly with performance.
Resetting your admin password - Forgetting your Administrator password can be a real pain when you want to install a new piece of software or run Apple's Software Update application. If you have a vague idea of what the password is you can try entering it, but if this doesn't work you will have to boot the Mac from the OS X install DVD that came with it, by restarting with the disc inserted in the optical drive whilst holding the [C] key down. Then select Utilities > Reset Password and choose a new one. When you reboot the new password should be working.
Fixes for viewing WMV files - The web is awash with video formats and not all of them are particularly Mac friendly. The one you're most likely to encounter is Windows Media Video. Download Flip4Mac, a free codec that enables the QuickTime Player to read WMV files. Alternatively, download the VLC Player from the disc. It can play back many weird and wonderful video formats, though it's not the world's most elegant piece of software. Finally you can install Perian, which provides QuickTime with a raft of codecs to help it open other kinds of videos you may come across.