Excel 2003 (11.8117.8107) sp2 on XP desktop

Out of the blue, a user is reporting that with every Excel file opened, a
new instance of Excel.exe is started. Closing the file doesn't end the
instance - she must Exit.

I can't find any info on this - what setting might have changed to cause
this to occur? Or might something outside of Excel be causing this problem?

It only happens in Excel - not in Word or other Office products.

It doesn't matter if the user opens the files from our DMS (Mailsite 8.0) or
from within Excel.