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When first going to install the MSDN download once you burned the dvd most likely you could easily reformatted C or simply used the custom option to replace the existing RC installation there with the finished 7. With the custom install to the second partition the boot entry for the RC was added into the BCD store as part of the installation there.
For removing the RC at this time you would risk the loss of the boot files the Windows installers placed on the root the C primary. That would prevent the second retail install from loading up. Your best option is to either start off fresh if you have all files backed with a reformat and another clean install on a fresh C primary or use the custom option and end up with two installations of the retail version you downloaded.
The last option would simply using a drive tool and reformatting both or removing the partitions entirely to create a brand new single large primary for 7 to go onto. The reformat of both would leave the second D partition for backup and storage if the drive is large enough.
For reformatting purposed you will need a 3rd party tool. For a fast and effective free drive tool to see that done you can burn a iso image of the Linux drive tool GParted onto a blank cd-r and boot from that to simply right click on the paritions one at a time and choose the format option. You will have to select NTFS from the dropdown choice list since GParted covers multiple OSs.
The latest 0.4.6.1 release for that is available for download at
GParted | Get GParted at SourceForge.net
Read over the general information and instructions for use on the documentation first however.
GPARTED DOCUMENTATION - GENERALITIES
You should already have the program for this since the 7 downloads are in iso images. However if you have a usb flash drive which can be a 1 or 2gb model for GParted you can also create a usb key to boot from for use. The latest ubuntu live version 9.04 also has GParted included with that as well.
