It's has an extension of .xlb, but the file name and location can vary.
For me (xl2003 and WinXP Home), it's in:
C:\Documents and Settings\(username)\Application Data\Microsoft\Excel
And it's named:
excel11.xlb
(excel eleven.xlb)
With excel open, make a minor modification to a toolbar (to find "true" toolbar
file).
Then
Close excel
use windows search to look for *.xlb
and delete all that you find (or move them to a safe location if you're worried)
And remember to look through hidden folders, too. People can have trouble
finding them.
If you can't find your files using windows start button|Search, then you could
use an old DOS command to search and delete/move them
Windows Start button|Run
type:
CMD
(to get to the Command prompt)
C:
(to get to the C: drive--change this to the drive that excel is installed on)
cd\
(to get to the root directory)
dir *.xlb /s
The /s says to search subfolders, too.
Keep the *.xlb file that was just changed and delete the others.
(There could be multiple *.xlb files depending on your excel upgrade path.)