Ok.. you cant downgrade like you may think.
But you can run different versions of windows on the same pc.
Its called Dual-Booting and if you want you can even triple boot with XP, Vista and Linux
Heres a link:
Windows Vista Beta: How to dual-boot Windows XP and Windows Vista - Lifehacker
Basically its just the same as installing a new OS but before you add-on another OS you have to make a partition on the harddrive (seperate the harddrive into parts) so that that OS can have its own space.
(Edit) The two OS's wont run at the same time, your computer will just give you a choice of which OS to run at each startup