Legodude, I'm totally with you on WinXP. However, I recently installed Windows 7 and I'm glad I did. I'm not going to try to convince you, I'm just saying I know where you're coming from but Win7 is worth installing IMHO.
Your compter get slow when your ram is get crowded. When you run a program it loads in the RAM. So always try that do not run too many programs at Startup.
Atomic Rooster is 100% correct. The only exception is when your hard drive is almost full. Then it can slow down. This is because Windows requires a certain percentage of the hard drive to be empty. As long as you have at least a few Gigs of available space, you will not experience any slow-down.
Definitely adding more stuff on your computer than the usual won't make it slower. I believe that those unwanted files and programs and those we usually downloaded from the net slow down the performance of our pc. I've experienced that so many times and it's kinda irritating. Good to know I found pc health advisor serial because it clean up my window registry and removed those files that are needed or looks like malicious at some point. It's better I guess to check the performance of our pc once in a while to avoid the slow.