This is discussed in the divx.com faq’s. I’ll paste the URL at the end of this because it’s *ridiculously* long (blame the divx.com sysadmin).
Go to divx.com. Click support on the very far right. In the box search text type “upside down” without quotes, then click search. The answers you want are 1, 3, and 4. I don’t know what to say about unregistering directshow filters, except that I wish they had an undo option mentioned somewhere but I didn’t see one. In any case if all else fails then that’s what you’ll what you’ll have to resort to.