Computer freezing every couple of seconds and programs not responding

For the past couple of days my computer has been freezing every time I open a new program or go to click on something. Using itunes is especially problematic because i will be listening to music and every thirty seconds the music will stop and continue, etc. I use Avast! free for protection and I have done every scan without them finding a virus, however they do get a couple of files they cannot scan and each time they are different files. I am not very tech savvy so I have no clue how to go about searching for the fix, however while searching through google i did find a lot of people had this problem with windows xp.

I own a Sony Vaio VPCEG32FX/W and have 142 GB used memory.
Programs i have installed are Photoshop CS5, Finale Notepad, Itunes, Last.fm, Firefox, Windows Live Messenger, The Sims 3, Avast!, Avidemux 2.5, Rollercoaster Tycoon, Winrar, and VLC mediaplayer