OpMangr SQL Installation Problem

Hi Folks,
I'm having a problem getting OpMgr installed on a server which currently runs an instance of MS SQL 2005 Express for a different application (I'd rather have one MSSQL instance instead of one MSSQL and another MySQL both on the same server).

I'm getting the error "Unable to connect to SQLServer. Proceed the installation with MySql as database". I've verified that MSSQL is set up for TCP/IP connectivity and is listening on localhost:1433, I've also verified the logon credentials are correct (domain user with MSSQL SysAdmin privileges). I've left the DB name as the default "OpManagerDB", and have tried manually creating a DB named this in the MSSQL instance but still no luck. Am I missing something obvious?

Thanks,
Alistair