If you wish to dual boot Windows XP and Windows 7 RC than you have to make sure you install them in the proper order...

You have to install from old to new.. So the proper way would be to install Windows XP FIRST than Windows 7 RC SECOND.. The boot manager will configure itself as long as you install in the right order..
I would suggest you do a clean install of XP over top of the Windows 7 installation you just did.. then do a clean install of Windows 7 RC on a seperate partition.. You will then have a dual boot setup and as I said the boot manager will configure itself and you'll be presented with an OS Selection screen each time you boot up..
To configure the boot manager to your liking after you have both OS's installed...