|The best way to do it would be to wipe the HDD & start from scratch. But before you do, make sure you have ALL the necessary device drivers (motherboard chipset, video, audio, network, wireless, etc) for both XP & Vista.|
If you want to create a dual boot setup, partition your HDD into at least 3 separate partitons...one for XP, one for Vista, & one for data storage. Install XP on the 1st partition. Once you have it installed, pop in the Vista disk while you're already booted into XP. A window will popup asking if you want to upgrade, however that is NOT what you want to do...instead, select "advanced" & direct Vista to install on the 2nd partition. The boot menu will be setup automatically.