If you’re one of the (presumably very small number of) people whose Media Center installations were hosed by Rollup 2, here’s a fix: MceRepair 0.2 (alpha). (Update: As of a few days ago, there’s a new 0.2 version.)
The author is a Media Center developer, but the usual disclaimers apply: This is alpha software. It’s not officially supported by anyone. It could make things worse, not better. Don’t run with scissors.
(Thanks to Ian Dixon for the pointer.)