Microsoft released a first release candidate of their upcoming Microsoft Vista operating systems a few days ago. I don´t think it´s wise to install a beta version of a microsoft os as your main operating system:dual booting is the solution. You want to experience Vista and still be able to return to XP or whatever other operating system you are using ? Fine, follow this easy to understand guide to dual booting.
First, download Vista release candidate from the official microsoft website and get your key. Second, get the gparted live cd from sourceforge. Use gparted to create a second primary partition. Vista apparently needs a primary partition and wont install on others. Make sure you assign enough disk space to that partition, recommended is at least 20 Gigabyte of Hard Disk space.
If everything works out you now have a second primary partition which is ready for windows vista. Reboot your machine and load Windows Xp once again. Now run the windows vista setup again and chose your newly created partition as the target for the operating system.
Once the installation is finished you should see a bootup manager after turning on your computer.
The installation should be similar if you are running linux instead of xp. Just use the live cd and create a primary partition and install vista. (boot and setup from burned dvd if you are running linux).