what spec of PC is it? you realistically need 1 gig RAM min. if you haven't then you need to get a RAM upgrade, I would personally recommend Crucial
Memory upgrades, graphics cards, flash media, and usb storage at Crucial.com
Aim for as much as you can afford, 2 Gig seems to make it a lot better than 1 gig from people I have spoken to.
You should also consider turning off all the crap that Microsoft set to start as default. There are about 15 or so services that are set to run that 90% of people don't need(they are eating memory/cpu). have a google I found a decent article that tells you which ones, if you can't find it let me know I'll have a quick look