Everything Staigermanus mentions is what I would recommend.

Just to give you an idea.

A Clean Windows XP Machine install should only display 16 Tasks running inside task manager.

The machine I'm typing on, is runningn NOD32 and I have a total of 20 Tasks running.

When installing software, when possible I choose custom and I make sure I understand what it is going to install on my machine before I go ahead and click ok.

Memory and CPU Resources can easily be lost with just a few installed programs.  Whenever possible.  Disable the following.

• Checking for Updates  other than my VirusScanner, I dont need Adobe Photoshop or My PDF Reader to check for updates.  I normaly check for updates manualy if I think there is a need too.

• Disable start when windows starts when possible.  I prefer Trillian over MSN Messenger, but yet I dont allow it to run on startup... if I feel that I need to CHAT with someone, than I will start the program otherwise I dont need to have it sit in my tray while I want to do graphics work.

• Any sort of Quick Launch - some software like Microsoft Office will load these Loaders that are supposed to help load their software faster when you need to launch them like Word or Excel.. the time savings with these loaders is marginal and you dont need them.  I can wait an extra 5 seconds to load Excel or Word.

I'm amazed at some of the machines that I come across that need optimizing and have 50-60+ Tasks running.

Some Common Resource Hogs, that are un-necessary.

• Instant Messaging Software, 
• Printer Assisting Software (i.e.Epson, Lexmark)
• Microsoft Office Loader
• Nero <--- Driver Compatibility Checker.
• SUN Java Update Checker
• Any Preloaded PC like HP, Dell, Gateway will have these update checkers or Customer support assistance running in the background, total waste.
• iTunes, RealPlayer, Apple Quick Time <---- GRRRRR!!!!

you want RealPlayer and Quick Time??  Google for  "Real Alternative" and "QuickTime Alternative"  both use Classic Media Player as a warpper and do an excellent job replacing the original bloated software.

• Photoshop <-- Checks for updates on startup
• Adobe PDF <-- Checks for updates on Startup <--- one of the worst PC Resource Hogs.  v5.5 was the last Best unbloated version.
• Skype <--- too many tasks to mention
• Logitech WebCam <---- as much as 6 tasks running for this junk.
• ATI Video Card Catalyst Drivers <---- You dont need all the extra wiz bang, just download the driver only portion and install.

You will notice to ATIXXX tasks running if you have installed the Catalyst... these tasks poll you're PC looking for special Keyboard Shortcuts pressed for Special ATI Commands...which I dont know what they are dont care about.  You need to disable these from the "Services Menu"
 
I can go on and on with this... :) But you get the idea.