General Lagging - Computer Drivers

i'm running Windows XP Home Service Pack 2 Edition, with all the latest updates, all my drivers are up to date but when I preform general tasks such as web browsing my computer lags badly.It takes forever to log into Windows and takes even longer to load Firefox up. Sometimes I get the (Not Responding) appear in the title bar, and the whole computer will lock up only to suddenly come back to life in a minute or so.It's been doing this about a month after I got it, but when Windows is first installed after a format it runs as smooth as a whistle then after a hour or so it starts to lag. I doubt its the RAM as im running 1GB's worth and the computer isnt that old I only got it brand new last year.