Eventually it sounds like you need to reformat and start over. Save your important work and start over. This is the 'if all else fails' step.

Before I did this I would also check for viruses. It can give you false information like this.

Also I would run system file checker and see if any files need to be replaced. Start>Programs>Accessories>System Tools>Microsoft Information>System File checker.

IT will prompt you if any files need to be replaced and ask you to insert your disk.

Good luck
Orangman