Best thing for you to do is the following...
open "My Computer", Right click on your C drive and select properties. Then when the window opens, click on Tools. At the top of the new tab that opens is Error-Checking. Click "Check Now" and when the new window opens, check both boxes and click start. A new window will come up, just click ok, then reboot the machine. Let it run the check disk and do what it can. After that, you can work on the machine some more. You probably have a few bad sectors. This can fix that for you, for the short term. Once your machine is done and you can get back in, there is something that I will want you to post, from the event log. This will tell you the condition of your machine.
Good luck