My first instinct is to reinstall the whole thing. Can you tell us if it has just started happening or if its been doing that? Another guess is that if it has just started happening then you probably have a virus, hard drive corruption, or you installed a patch that caused it. So my suggestions would be to do the following:
1. Reinstall, but get your save files first
2. Run virus scan
3. If you have another hard drive try to use it instead, but only do this if the other two don't work
Good luck!