Technically, I believe this would be a violation of the EULA, but I don't know for sure. What would be the reasoning to install the same OS twice on the same computer?
If you want a backup, use a drive imaging program, such as Ghost.
If you want to keep one install slimmed down for gaming, you can do this through profiles and user accounts, without needing a second install of the same OS.
If you wanted a second install to test software out in, use Virtual PC 2007.