Are we talking tower or laptop ? If it's a tower I recon the easiest and best way is to have each OS on it's own drive. With 1 drive plugged in install XP. Once up and running unplug this drive. On a second drive install Win 7 and get things up and running. Now plug both drives in and set your prefered OS as default so when the PC is switched on your prefered OS boots automaticly. To boot into the second OS at post use your F button, in my case F8 to select the second drive. Maybe a little long winded but works perfectly.