Actually you can just plug in a 2nd drive and install an OS onto it. Only problem with installing XP after Vista is it overwrites the Vista boot loader. You have to repair the Vista boot loader before you can boot back into Vista, then you have to manually add XP to the Vista boot menu.
There's a
sticky at the top of this forum about that, and here is another article that shows
how to use Easy BCD to fix things.
If you remove the Vista drive and install XP, you'll have modify the boot.ini file so XP will boot as well as adding it to the Vista bootloader. And in that case, each OS will see it's drive as Drive C and the other as Drive D, which may be good or bad. If you think you might want to remove Vista at some point and keep XP, this does have an advantage though, as the XP startup files will already be on that hard drive.
It would help if you can post the actual stop error message, including all the numbers and any file names that are mentions. What you posted was just the things to try.
Common problems:
Using an XP disk that does not include SP2 on a system that has PCI Express slots. It will give a blue screen.
If the XP disk does not have at least SP1, it will only be able to see 137 GB of the drive.
If you are installing onto a SATA drive, you'll either need to change the BIOS setting for the SATA controller to ATA mode (which may cause problems with the Vista install) or slipstream the SATA drivers into a new CD (If your system has a floppy disk drive, you can provide the driver on a floppy during the install)
Newer systems may not have XP drivers for all the hardware. You may find you have no sound, or limited Graphics capability. Best to track down all the needed drivers before hand.