Did you guys even read the title? He wants to install OFFICE(XP Edition), not Windows XP.
Go to Start | Control Panel | Programs | Classic View(upper left corner of window) | Programs and Features. Find Microsoft Office in the list, right click it and then click uninstall. Follow the instructions, Restart the computer after it is uninstalled.
After your computer boots up, put in the Office XP disc, popup should come up on the screen, click Autoplay or install(whatever it is), follow the instructions to install it, should be pretty straight forward. Restart computer after it's done.
All set!
On a side note there's nothing wrong with Vista besides the fact that it is a memory and processor hog. You gotta have a good machine to run it well. Machines with 1GB RAM, 1.6 Ghz processors and such should NEVER have Vista on them. Microsoft is shoving it down all the OEM's throats though.