[Gravter]

[puts on nerd hat]
I reformat my computer almost every year, no matter.
I would start with that if your comfortable with it and see if you notice any change in speed.
RAM is always good, but it depends how much you have right now and what applications yours using (simultaneously that is).
I always recommend 512MB to 1GB RAM for XP (mostly for internet users) and up to 2GB's for more advanced users (photo/video editing, games, etc)
Is your hard drive saturated? If you have less than 25% left, it can really slow down because it has to fragment files when creating them. So another option is to do a defrag of your hard drive. Even getting a newer drive is sometimes a good idea (read: replace your old drive by ghosting it to your new drive).

There are so many variables it is sometimes hard to pinpoint. So if you can provide more info on your personal confuser, it might be easier.

[/removes nerd hard]

Good luck