My laptop has been doing the same thing , working fine then grinding to a halt ! it has only been doing it since visiting a few new porn sites . I ran a de frag , and cleanup , downloaded and ran the newest version of ad aware and norton and it all seems ok now .
Seing as i know fook all about computers this information is probably about as usefull as a one legged man in a bum kicking competition but you might have visited the same sites accidently like i did