Can't install MSSQL 2008 SP2

I'm trying to install SP2 on a SQL 2008 server. The setup runs, passes the prerequisite check, and we accept the license. Setup shows some of the installed pieces, but:

1. It reports that there is no instance to upgrade

2. It won't let you check the checkox for the pieces it found.

I'm doing this as an administrator, and am running the setup with admin priveleges.

Any thoughts?