I agree with Mike. It sounds like the HDD is about to die. If you go to 'My computer' right-click on the C drive (or which-ever letter denotes your drive) and click properties. Along the top choose tools and schedule a disk scan, make sure you click on all the relevant boxes. Reboot and the checkdisk will go ahead unless you stop the prompt.
You could also try this utility called HD tune it will check your drives using SMART (might have to be enabled in your bios) and will look for errors. Its free and you can get it from here: HD Tune 2.50 download from Guru3D.com

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