I figure I already paid for the pre-installed OS. In this case Vista.
As you know I'm sure ; even with a new computer they no longer provide an "install disc" legal or otherwise. I just cleaned out all the crapware & Vista has done better for me than most .
If I bought a legal XP install disc I don't know what BIOS change would be necessary.
As for the capability . This Lenovo is still running the original Vista OS.
With a lot of cleaning & other tweaks. Minus the "special" restore partition.
On the original harddrive.
It also now has XP Pro on it's own harddrive & Windows 7 Home Premium on it's own harddrive. I usually boot to W7 & it's not a "triple" boot.
When I want or need to use one of the other OS's I just F12 during start up & choose the harddrive I want.
I do know how to create a dual boot as my laptop has W& 64-bit & XP 32-bit that is dual boot. I've never attempted a "triple" boot but I think it's possible.