graphics problem after blaster, can anyone help?

Hello there, I am running a laptop with a 1.7ghz P4, ATI Radeon
Mobility M6, and 256mb RAM. I have been playing basic 3D games for the
past year such as Half-Life, Quake1,2,3 etc, and have had no problems
at all, smoothe and fast running as you would expect. Then I got the
blaster worm, and welchia, and having removed them completely, patched
and updated XP, and updated my anti-virus etc etc, i have had major
problem with graphics which im not sure is related or not. When
playing a game, (online or offline) it runs smoothly for abou 50
seconds and then gets a slow down and everything goes jerky. It goes
back and forth from smoothe to jerky throughout and I cant seem to get
it sorted. It seems like something may be clogging up the processor
but I dont have any suspicious processes running. I have since
formatted my computer.....and like a fool I plugged my broadband back
in and got the welchia and blaster worms again!! They are gone now but
the problem is still there. Can anyone help? sorry about the long
post, im new to this.