First off make sure you download the RC from Microsoft found
here. Make sure you burn the ISO at the slowest speed. Secondly, is this the result of trying to upgrade as opposed to a clean install? My advice if you are wanting to try out 7, is to do a clean install on another partition, or better still another drive and dual boot with Vista. That way when the RC expires you can always go back to how you left Vista.
There are lots of guides on this forum, but basically you need to shrink your partition and boot to the DVD and install 7 on the unallocated space.