Try beefing up Virtual Memory.
Control Panel > System > Advanced > Advanced >
Under "Performance" click "Settings" click the "Advanced" tab.
Under "Virtual Memory" click "Change"
Select your drive and choose the radio buttom "Custom Size"
Set it to around 4,000 - 4,000 (i think that's the max on XP anyway)
ONLY DO THIS IF YOUR HARD DRIVE HAS THE SPACE FOR IT. 4,000MB
AND ALWAYS MAKE SURE YOUR HARD DRIVE KEEPS AT LEAST 2GIGS OF FREE SPACE AT ALL TIMES. If changing this setting cuts you anywhere near close to running out of space; buy a new Hard Drive.
Then click "Set" > "Okay" > "Okay" and if you're not prompted for a restart...restart anyway.
Also, I'd investigate for Spyware. Whether you use SpyBot or SpyHunter3 - the two programs I'll stake my life on - investigate any Spyware, Maleware, Virus', Trojans, Wyrms or, *censored*, PDQ Speed-up on the internet is to clear Caches, Cookies and stored data like Login Information and other such nonsense.
Future Suggestion; if you do buy a new HDD and you know what you're doing while installing windows; Make yourself a separate partition for the OS (30G is awesome) and leave the rest for installing programs and saving data. Just make sure any time you install a new program to point it towards your second partition. Works great for me.
That way if I have performance issues I just format the OS partition.