computer running slow and freezing. no evidence of virus

[romeoagogo]

Hi, my roommates computer began running very slugishly recently and we cannot figure out why. I installed norton and did a one button checkup and fixed all the errors and I ran a NOD32 antivirus in depth analysis which returned nothing. I did recently use GParted to repartition his drives but I don't see how it could have cause an issue like this. I checked task manager and it doesnt show any odd applications running but when I check the performance tab it says 30-40% CPU usage. I am about ready to a fresh reinstall but just wondering if anyone had any ideas.
Thanks