you need to find out what you really need to run things all the time and what is only needed for special events, and what is user-specific, or not necessary at all.
One way is to load Spybot (if you havent already), and click on the tab (under tools) called "system Startup". this will show you exactly what your computer is running in startup, and most if not all of those will have a sidebar telling you what you need and what' unnecessary.
If you have programs that you aren' sure of, type in the name of it at Google, and find a site that will tell you about them, one at a time. You'd be amazed at the junk you can accumulate. And you dont have to delete any of them, just uncheck them in Spybot and see if there's a problem. If there is, go back and recheck it.
If you run XP, you can disable Prefetch totally, and never miss it.
clean out what's in there, and you'll be amazed at how much more quickly things will boot up.
All prefetch does is cache the files you visit, so that in theory they will load faster. What actually happens is that it doesnt empty that cache, and it keeps getting more and more bloated.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic