You dont need to change any BIOS for this but instead download Paragon
Drive backup(trial version will do) then make a boot disk for Vista (for security measure). Then also Download Easy BCD install it in Vista. Ok now install WinXP. In WinXP try to install easy BCD if it shows two entries for OS then it is ok but if it doesn't then boot through your bootdisk vista will open up open easy BCD search for OS and add to MBR.
Paragon-http://www.paragon-software.com/home/db-
Easy BCD-http://neosmart.net/dl.php?id=1
