Use Windows Virtual Machine or VMWare.
As long as the computer meets the system requirements for Windows Vista, yes.
Yes. "Vista ready" implies the PC has no operating system installed already. It will have a higher specification of components (i.e ram, hard drive, graphics card etc) than is required to run XP.
Yes. You can dual boot both operating systems at once. Or you can install Windows XP over Windows Vista
The only way is to get a windows XP CD and do a fresh install which will completely erase all your data from vista.
Yes