you just want to delete a single cookie, right?
IE cookies are easy to find. they're in C:\documents and settings\user name here\cookies
make sure you don't just click on the my documents link because that only takes you to the my documents folder, which is in the folder documents and settings.
the individual files are named [email protected]
if there's lots of them you might want to do a search based upon the domain name of the forum but most of the time this isn't needed.