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#11
Boot the Vista DVD, select Custom install, select C partition where Win7 is now installed, use Drive Tools (advanced) to format first, then install Vista there. It will update the MBR in XP to make a dual boot.
If you want to install Vista to the empty H partition, it will configure a multi-boot with XP and Win7.
In either case you should not lose the ability to boot XP as it will retain the MBR. If an OS is missing from the boot menu, we can help you add it using EasyBCD 2.0