Run anti-virus as Sephire said, I woud download a couple different free ones and run them all (some pickup stuff others won't)
If it finds nothing, try running a check disk -f on the hard drive. 
After that run Disk Cleanup and a Defrag. 
Another thing to note, how much free space do you have on the hard drive? As a drive approaches it maximum capacity you will start to noticed degraded performance and freeze ups. 
Hope that helps.