I believe that XP must be installed first.
You can use the XP installation program to setup your partitions.
Boot into the XP installer, then use the disk partiton screen to delete all partitions. Repartition as you see fit. Install XP.
Once XP is installed your should be able to install Vista on the other partition. Vista's boot loader will override XP's so everything should work well.
Edited by Amazing Andrew, 16 March 2007 - 09:34 AM.