My Windows XP Check List
How to keep windows running reasonably well with few problems

Install a fresh copy of windows from scratch

Download Mozilla Firefox from http://www.mozilla.org/ set as default browser,
_never_ use IE again except for windows update. In FireFox go to edit/prefs
advanced, and expand the software update tree, uncheck "Allow web sites to
install software"

Windows update, Windows update, Windows update, reboot, reboot, yada yada
do the driver updates and optional updates too.

Get latest Macromedia Flash Player from http://www.macromedia.com/,
java from www.java.com

Install SpyBot S&D from http://www.safer-networking.org/en/index.html
Make sure to install/enable TeaTimer. Unless you are doing updates always
click "NO" when teatimer asks if you want to modify something! Use this program 
every once in a while to check out your system, and be sure to immunize. 
Put it in "Advanced" mode, go to tools/startup list and look through there,
delete anything that doesn't belong (such as the Sun Java Updater)

Install whatever software you normally do, I recommend Winamp over MusicMatch,
and media player classic + ffdshow codec for movies. Do NOT install kazaa
or any other such spyware-ridden crap (Ares is a good alternitive). If you IM
on yahoo+AIM, try out Trillian or GAIM, much better than having 3 IM programs.

Get Diskeeper trial from http://www.execsoft.com (careful they spam!), install,
run, and set a boot-time defragmentation, reboot and let it do it's thing.
Do a set-it and forget-it schedule, and uninstall when the trial period is 
over. Start doing manual defrags instead (Accessories/system tools)

Go to start/My Computer, right click on it and go to properties, go to advanced
and click the settings button for performance. Turn off as many of the 
visual effects you can to make your system more snappy. I usually uncheck
everything except "Use visual styles"