I have a very annoying Windows XP Explorer bug. The "Type of File" drop-down list (under Search-More advanced options) does not contain the first entry: (All Files and Folders). When I launch and do a file search, this field is blank and the serach works as expected. However, at the end of the search, Explorer places the top value in the list in this field (happens to be 3GPP Movies) and I cannot remove or delete this so all subsequent searches are restricted to 3GPP files. To continue normal searching I have to close and re-launch Explorer.

Any suggestions appreciated.

You can try a sfc /scannow at a run prompt
This will check your system files to see if there are any corrupt or missing system files
You may need your installtion disk