I've edited thousands of tunes using my Windows XP computer. Recently purchase a new computer using Windows 7. Immediately with the first file I try to correct ( the artist name) I get a "Cannot be renamed/Access Denied" Message. I'm doing nothing different. Can't find the answer I'm looking for in FAQ. Merman
This has nothing to do with MP3tag but with the access rights on your system.
To confirm that, switch to the explorer's "details" view and display the column "owner" - this should show either somebody else or a strange combination of numbers and letters.
To become owner of these files, use the windows means that you find in the "properties" of the top-most folder in "Security" "enhanced".
If you are not sure where to look to modify the "owner", consult the microsoft knowledge base
Also, try to open your music files with the means of folder selection from within MP3tag and not by drag&drop as sometimes the indexing and stuff block files.