First, stop using the CTP. It's not supported and RTM has been released. Move to that. Likely it's a VS2008 conflict somewhere, which is why I'd say move to RTM.

I'd also consider moving to VMs for this stuff. Set up one with Windows and each version of SQL, installing VS in each one as well. You can fire them up as needed and they'll prevent weird conflicts.

Virtual PC is free to use here.