We've never dealt with the SQL Server compression feature professionally, but we got a call yesterday from a consultant that noted that a customer had accidentally turned on compression in a SQL Express server while installing a service pack. At least that's what the customer stated.
We didn't even know that compression in SQL Express was possible, or that a service pack would behave like that.
None the less, the consult found the script below and ran it against the server, and it corrected the problem.
Use this with the greatest of care. Back everything up. Test, test, test.