I do not like 7. I am elderly, bought a new pc and 7 was installed. I hate it! I do not need all the bells and whistles. I see I can install XP but want to be sure it's okay. Sometimes us oldsters have a thing with learning new hi tech stuff!
The safest way to do this is without blasting your Windows 7 system out of the water is to buy a new hard drive (doesn't have to be large), install it in your computer, and install Windows XP to it.
But I suspect you may be talking about adding an XP virtual machine to Windows 7 - that runs Windows XP "inside" Windows 7. I have a feeling you'll probably find it even more clunky to do things that way than just to use Windows 7 on its own.