You can use a boot loader program to allow you to have multiple operating systems. I use EasyBCD to do this. For example suppose you have Windows 7 installed on one drive. The you add another drive with Windows XP on it. To create a boot menu just run EasyBCD. You can then easily add a option to boot Windows XP also. When you boot with a boot loader menu you are given a menu to choose which operating system to load.
EasyBCD is not necessary if you install Windows XP first and then Windows 7 on a different partition or drive. The boot menu is created for you. Otherwise you need a program like EasyBCD to create the boot menu manually.
Note all operating systems must have been originally installed on that computer so that the drivers are correct. Also you don't have to install different operating systems on different drives. They just have to be in different partitions. For example on my laptop with one hard drive I have both Windows 7 and Windows 10 installed.
BTW, one time I had 3 or 4 operating systems at one time on a computer. Two were versions of Windows while the others were Linux. I just selected which one to load from the boot menu.
EasyBCD
http://neosmart.net/EasyBCD/
Note EasyBCD is free for personal use.