If you still have the RC and license key, then just boot from the DVD/flash, select "Connect to the internet during install" to get important Updates during install, then Custom, then Advanced drive tools to Delete, create any New partition(s), and Format before install.
In March, the RC will start shutting down every two hours to remind you to move on to a clean install of purchased Windows 7, until early June expiration date. If you boot from the DVD to get the cleanest possible formatted install, it will scan your HD and see the prior OS to allow you to use an Upgrade key.