I've downgraded to version 2.32a, and am having no problems now. This looks a like a regression to me. Florian?
Weird, I am using the most recent version and I had no problem tagging a 42.2 MB MP3 file a few seconds ago (ID3v1 and ID3v2).
Hmm, well, it isn't working for me. I guess I'll be sticking with 2.32a until I can figure out why...
Yeah, sorry. 
I've just finished the installation of a brand new Windows 2000, reproduced the bug and will fix it to the next release.
Thanks for your patience.
Best regards,
~ Florian
Excellent! Thanks for giving this your attention, and keep up the great work 
This bug is fixed with the current Development Build.
Best regards,
~ Florian
This is a rather late reply, but it might help for future problems. If a file is open by an application, one can find the application by the use of the command line utility "Handle" or the GUI program "Process Explorer" from sysinternals. The programs are freeware.
Just check out the following site:
Sysinternals Freeware
Follow the link to the program "Handle" or "Process Explorer", and read the instructions there.
Best regards, Kosmas