Can't access media files after renaming them

I have a recent problem with media files (MP3, MOV and AVI). After
renaming the files in Windows Explorer, they will not play. Nor can I
move, delete or rename them a second time.

I get the error message "Access denied. Make sure the disk is not full
or write protected and that the file is not currently in use."

The files are NOT marked read-only and no app has them in use. The
problem only occurs after I have renamed them, keeping the original
extension. This happened with MP3 files created with Total Recorder,
with QuickTime videos shot with my digital camera and with AVI files
uploaded from my camcorder using Roxio.

I'm running Windows XP Home on an Athlon PC. I don't have the problem
with text or graphic files.

Any suggestions would be warmly welcomed.

Ric