could be the hard drive starting to fail. If your getting read errors on the screen like 'Error reading drive c, or sector not found' it sounds like the hard drive is dying so you need to get it recovered (you important info off it and onto a replacement drive) by someone like a technician. make sure no cd is in the drive and try booting again. if same problem, get the hard drive replaced. to get this done you will have to bring it to a techcie to fix it. I am based in n ireland and do this sort of work regularly. If you want i could fix this for you.
martin devlin
September 2007