Well that's a bit short-sighted, isn't it?

As long as you have a licence for XP, and the relevant XP drivers for your
hardware, then of course you can remove Vista and install XP...

How to Downgrade from Vista to XP:
http://www.pcmag.com/article2/0,2817,2287685,00.asp

Probably won't be the easiest option, I must warn you.

Check with your computer seller. Normally you have to downgrade to xp
yourself and buy another license for xp. Before you do that, try
Compatibility Mode in Vista. Look it up in Help.

The date and time was 5/20/2008 8:43 AM, and on a whim, stickerman
pounded out on the keyboard:

Removing Vista will most likely void your warranty.

As alternatives, you could possibly install a virtual machine on the
Vista workstation and install XP into it to run the software.
http://www.vmware.com/
http://www.microsoft.com/windows/products/winfamily/virtualpc/default.mspx

If you absolutely don't want Vista, I would buy a copy of XP along with
another hard drive.  Remove the existing hard drive and store it for
warranty purposes (or leave it in the machine and just disconnect it),
install the new hard drive and install XP.  The new workstation driver
disc may not have all the drivers needed for XP, so that might be a
problem also.  You could dual boot as another option.

1 Some computer manufacturers give downgrade rights to XP, if you have bought
vista business; check with yours.
2, Before you do anything check with your comps manufacturer to see if XP
drivers are available for your model computer.
If no XP drivers, forget it!

I would talk to the seller and see if they offer XP as an option.
It's getting harder to find OEM machines with XP, many I see are expensive
extreme gaming machines not the everyday user's normal choice.
Have you tried installing the program in compatibility mode?

  It's your computer, so the choice is, ultimately, yours.  (Although
it would have been a lot simpler to have just purchased the computer
with WinXP installed, to start with.)

    However, there could be a couple possible adverse repercussions of
which you should be aware.  First and foremost, if the specific computer
model in question was designed specifically for Vista, there may well be
no WinXP-specific device drivers available to make the computer's
diverse components work properly.  Consult the computer's manufacturer
about the availability of device drivers.  Secondly, removing an
OEM-installed operating system and replacing it with another will almost
invariably void any and all support agreements and, sometimes, even the
warranty.  You would, at the very least, have to re-install Vista before
getting any support from the manufacturer.  Again, consult the
computer's manufacturer for specifics.  Thirdly, there may be the
additional cost involved in purchasing a WinXP license for this new
computer.

    After backing up any data you wish to transfer to the new OS
installation, simply boot from the WinXP installation CD.  You'll be
offered the opportunity to delete, create, and format partitions as part
of the installation process. (You may need to re-arrange the order of
boot devices in the PC's BIOS to boot from the CD.)

HOW TO Install Windows XP
http://support.microsoft.com/default.aspx?scid=KB;en-us;316941

http://www.michaelstevenstech.com/cleanxpinstall.html

http://www.webtree.ca/windowsxp/clean_install.htm

    Then, assuming you were successful in obtaining WinXP-specific
device drivers so that the computer can be made to work with WinXP, the
backed up data can be restored and applications (those that are
WinXP-compatible, that is) re-installed.