localhost/mssqlserver is a named instance. (local) defaults to local (I think 127.0.0.1)

The /mssqlserver isn't necessary for a default instance. You'd just connect to the Windows machine name.

If you want to reinstall, I'd save off your databases (ldf/mdf files) just in case. Run uninstall from the Control Panel. Note that there are multple SQL Server 2005 installation items. I think RS is separate. REmove them all, whack the registry keys (HKLM/Software/MS SQL Server) and then reinstall.