If you reformat your hard drive, follow prompts to install, then partition drive in half and install xp., then put vista in and follow the prompts to install vista in the 2nd partition.
There is an easier way though to create a 2nd partition from within XP, and install Vista, and the software to do it is free, and includes a walkthrough.Step One
download Simply MEPIS 3.3 here:http://iso.linuxquestions.org/mepis/
Burn it to CD as an ISO image
(if you need an ISO burner for Vista default burner you can download it here)32-bit version64-bit version
Boot from the CD drive, put Linux CD in drive and boot. It will run Simply MEPIS from the CD, but it will not install
Sign in as: root, password: root
Click on "Run QTParted"- that will open a box showing your partitions. The main Windows partition is usually /dev/hda1
Right click on your Windows partition, and choose RESIZE
In the box labeled FREE SIZE AFTER, type the size (in MB) that you want the new partition to be. That will create free space.
Right click on the free space, click on create
Select partition type (NTFS or FAT32), click OK
Click on file(top left), then commit- changes will take effect
Click close, then click on the K in the lower left of screen and logout, restart computer
remove CD and it will boot to windows, check your partitions
Put XP in and follow prompts to install on new partition