Just yesterday I noticed my programs in XP were opening slowly and also running slowly. My computer takes a long time to start up too. I searched on Google and tried CHKDSK to see if the hard drive was ok and it was just fine. I then defragged and after that the problem was still there. From googling I found that my problem could be issues with my RAM. I ran memtest.exe for an hour and it didn't find any errors. One example of my problem is when I use winrar to extract something it takes much longer than usual to do it. I noticed in Task Manager the CPU usage is between 0% and 3% when idle and around 50% when it's slowly running a program.

Anyone have any ideas?

Edit: I forgot to mention I did a virus-scan with Norton 360 and a scan with Malwarebytes Pro.