So you're setting up to switch your operating system on both the computers. That's fine as long as you have the legit keys. They are on the CD holders or a label which is at the bottom of a laptop.

Just format both the hard disks previously to installing the operating system.

When you put the Vista disk it may or may not work for the basis you have confirmed. If it does work then be confident to have the accurate drivers handy as it is about sure that it will put the drivers for the laptop on the new PC.

Be confident to do the new PC first in case it doesn’t work so you can use the laptop to get online.

I have also noticed that Dell sells OEM versions of Vista and it is against the law to make use of it with another computer. Is this true?

It is only an illegal for installing that OEM license on more than 1 computer. Dell with the OEM license can be relocated to a different motherboard or even on a dissimilar machine, by activating through the Microsoft’s Telephone Activation, been there done that for people many, many times. The Win XP to the laptop also won't be a problem as long as all the necessary drivers for the laptop are able to be downloaded from the laptop website. The Dell Vista laptop to desktop though will absolutely be a problem if the laptop has a restore CD or DVD, that restore the disk will install all the laptop drivers, so if Vista can also even get you to the desktop, you'll have to have the appropriate drivers on hand and just prepared to install starting with the chipset M/B drivers first in it.

In my opinion the laptop to desktop install I don't think will work, but if it does I'll be happy to be wrong for you, and let us know what take place?