I was installing a program a few days a and got an "error writing to disk" message. Everytime I start my PC i get a message that I need to run scandisk, but when I run a thorough scan, the PC freezes up about 25% of the way through the serface scan. This happens whether I scan from windows or not. I tried to defragment but it also freezes in the same spot. Any ideas what caused this or how to fix it?