If this is to be used as a business computer using dual booting. I don't recommend it at all. Especially from a new computer built to vista standards. Vista is a new (different) type of operating system. It was actually designed to be non compatable at all with XP and Microsoft has paid dearly for that mistake with having to support downgrading computers to use the XP operating system.

You see a lot of people using dual booting...but most of these are individuals playing around with it and do not need to depend on the computers except as personal preferences.

Finding drivers for the new computer that can be used to run XP may prove to be next to impossible for some time or the computer has been used to run XP as an operating system and the manufacturer already has them readily available. and I can't check for you as all you called it was a "high end computer". I doubt if it is compatable at all.
I am not saying that it cannot be done...only that it may not be worth it from a business aspect if it crashes all the time or locks up or what ever.

My suggestion here would be to repair the older XP computer and use it as a stand alone computer that uses the same monitor and keyboard by the use of a KVM switch

KVM switch - Wikipedia, the free encyclopedia

very reasonably priced and easy to use.
Then find an external network storage drive and set it up as a shared storage drive...do the same for the office printer. and have the networked mapped out to the different computers, the storage drive, laptop (can also share with all others through the shared drive ) and the single printer. You can also add a mapped out back up configuration of storage drives to handle back ups from both the xp and vista machines.

With the KVM switch you use only one mouse one monitor and one keyboard for the two computers. Switching back and forth between the systems is not a problem. Using the mapped out shared network drive allows the protected sharing of files and folders without possible damage as with dual booting unlike systems.

There are some shops that will talk you into the dual booting if you let them...I just don't recommend it for Business applications at all!

And in this aspect a crash from one computer will still allow you to work from the other one while sometimes dual boot problems lock up all the operating systems.

edit: this is not hard to set up yourself and to keep it maintained...this keeps business costs down.