For Windows 7 (probably similar going to earlier versions) click the start icon in lower left corner of screen. Then search for Backup. You should see something like Backup and Restore. Click on this and then look for create a system image or a repair disk.
You could have a bad disk. Take a look at GRC.com and read about spinrite. This will fix your disk if that is the problem but you would probably want to replace the disk if you think it might be failing.
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