Source: Walter Chomak's System Center Operations Manager 2007 Landing Zone
I’ve updated the OpsMgr 2007 toolbox with Walter’s Cleanup Tool for Operations Manager 2007.
This is a command-line utility that enables you to remove all the components of Operations Manager 2007 from a local computer in cases where the typical method of removing applications through Add or Remove Programs in Control Panel fails.
This is a must have tool and I in my experience I would use it even if the uninstall from Add/Remove Programs was successful as doesn't quite remove everything.
I posted about how to use this tool sometime on my blog if anyone wants usage details.