Windows 7 seems to be a little more beasty concerning access and locking of files. It seems like a good idea not to have any other Explorer window open while editing larger files.
Apparently the indexer and the preview generator grab the files the minute they detect a modification.
So, if you want to edit the files either ...
copy the path from the explorer and merge it into the address inputbox in the tag panel, then close the explorer
...or
use the built-in directory selection methods of mp3tag.
So far my theory (this is how I work and I have not yet experienced this kind of trouble).
Could you please give a feedback?