Does XP need to be installed in its own partition, or like NT/2000, can it be installed on the old OS drive and then dual boot to the old OS?Well, if it's anything like Windows NT or 2000, both: Install it to its own partition, AND dual-boot.
In fact, wouldn't it be EASIER to achieve a dual-boot setup, if you kept each OS in its own partition (in stead of having them step all over each other's configurations)?