How do you put windows xp and windows vista on two hard drives?

You can't. If your computer has multiple hard drives you need to designate one as a master and the other as a slave. The master (or main hard drive) stores and uses the preferrred operating system while the slave serves as primarily data storage. If you put even the same operating system on the hard drives the computer will either hang or crash because it will try to run "multiple instances" of the operating system because it cant differentiate which drive is the master and which is slave regardless of how they are hooked up. The same holds true for using different operating systems on the same machine (Vista and XP). Choose which operating system you really want, install it on the master hard drive, and designate the other as a slave (with NO operating system files on it whatsoever) - its the only way to do it. In order to use both XP and Vista you need two separate computers.