Hi Ham,
this one's got a handy page on the Microsoft website @
http://www.microsoft.com/data/mdac21info/MDACinstQ.htm
There's info there on rolling back, but since MDAC is a system component, unless you have a backup of all the files changed with the upgrade, you're either going to have to resort to backups, or a re-install.
Sorry.
Check the URL above for more info
S.
// it takes almost no time to rate an answer