You overwrote the RC and it is only because of this miraculous MS workaround can you roll back to a completely overwritten OS.
Had you used free Win7 Backup Imaging then, yes, you could reimage the RC in 15 minutes using the Win7 DVD.
But to be able to boot an uninstalled, overwritten OS is almost always out of the question.
So make the most out of one of the rarest exceptions to an impossible situation by using the MS KB to roll back the windows.old folder as SIW2 instructed above.
Good luck. Once you get it rolled back, I provided you with another workaround so that you can Upgrade in-place the RC back to Ultimate while saving all your programs, settings and files and having perfect performance.
Both of these are nearly unheard-of fixes to problems which would otherwise be impossible to fix.