Workbook protection mysteriously added
 

Using Excel 2003.

My boss and I have both had instances of workbooks becoming password
protection when we both haven't implemented it (I've used the feature
before, and I know how to use it). It prevents us from moving or
copying worksheets etc. I've tried 'null' passwords, password, etc.
but it doesn't release control

Is there a virus or corrupted Normal.xls or something that could cause
this? It doesn't happen to all workbooks we have used.


Any help appreciated, it has become a real pain in the