Run defrag, disk checker, empty out the temporary internet files and history, run virus scan, uninstall programs that you no longer use or havent used in a while, get windows updates...
start there and if that doesnt help start looking at your hardware
Best free antivirus program
AVG Antivirus
Best Adware scanner
Spybot S&D
you have to do a couple things but its free and it works great.