It would help to know what you think is "slow." Wes has a good fix for "my computer boots slow." Whomever suggested chrome might have a good suggestion for "my web browsing is slow." Defrag'ing will help on application startup times. It can also help if you haven't defraged in 7 years and you're using a large amount of your page file (in task manager, there's a performance tab... PF usage is on that tab. Unfortunately "large amount" is somewhat subjective).
What I think would give you fair bang for your buck is adding ram. Things have gotten more intense since '02, and you're going to be filling your ram, resorting to your hdd as ram when you run over. And that's slow. If we were to put it in human terms, I think it's like you looking a word up in the dictionary vs taking a cruise to Europe to look up a word in a dictionary over there.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820141166 - I believe that fits in your computer. If you're up for spending a little money, get 2 and replace what's in your box (it's a really easy thing to do). I think you'd be happy with the performance boost.
That said, a full, clean install is the best route. I still think you'd benefit from more ram, though. If you didn't want the hassle of backing up data, you could get another HDD for your fresh install, but that's getting expensive.