Sounds like it could be a hard drive failure to me, at least thats the first thing that comes to mind. Another possibility is a motherboard problem. I would make sure you get a good backup, and then try removing the drive that you've been using and try reinstalling windows on the secondary drive and see what happens. if it happens again, it isn't the HD, but if it solves your issue, its the drive failing. I have had drives fail that way.