It might be a low memory as so many files or programmed in your system, it can be a viruses infections.
Please try these,
1. see what was the last work you have done before problem happening and please try to remove it
2. some time RAM can't loads your system
3. it might be a viruses infections
4. you can try Safe mode to access to your system using F8 Key while booting for XP, Win7 etc
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Computer keeps freezing help!