The question above is very relevant. Have you done anything as far and changing indexing options or any other file modifications?
If you open Windows Explorer and highlight the C: partition, then type something in the search box, what happens?
If you go to the Control Panel and select Indexing Options, the window that opens has a "Troubleshoot search and indexing". Since the trouble shooter was suggested before, what does it show?
Do you run any third party software that involves file manipulation?
You might try rebuilding the index, but it may not help.