Try Programs-->Accessories-->Sytem tools-->System information.
In that program, use Tools-->System file checker. It may be that some Windows files have been corrupted and need to be restored. On a clean system (no viruses) which hasn't been abused (shut down mid-flow), the most likely cause is a hardware failure (motherboard, memory, processor or hard drive).
For a temporary fix, if the system file checker doesn't work, you may need a fresh Windows installation. But if the cause is a hardware fault, the chances are high that this (or worse) will happen again. Ensure your important files are backed up!