It is suggested that Windows 7 be installed on a different hard drive, rather than just a different partition....and that's how I installed it.
I can't give a better answer than that.
If you had Vista installed on a separate partition on the same drive...then it seems to me that you ought to be able to install Windows 7 the same way. Win 7 and Vista both use the same bootloader, which is unlike that used by XP.
The Win 7 bootloader will take precedence over the XP bootloader...and will reflect your Win 7 install as the default boot option. To boot into XP, you will have to manually select "earlier Windows operating system" when the system first boots and presents the boot options.
One tentative guide: http://www.maximumpc.com/article/howtos/ho...or_from_scratch