I have done this many times, James. Forgot to mention it on the phone this morning.
I got a Dell with windows xp Pro and all the Dell garbage. I booted with a windows 2000 disk and set the Dell up with the software I wanted, using some of the Dell disks for drivers. OK. So the Dell was set up.
Now, I had the Dell OEM Windows XP Pro disk also which came with the system. So I installed that on a home-built PC, the kind you build. Same situation--it installed perfectly, without even my having to key in the CD key. It then gave the 30 day activation notice and I activated it without a problem.
Another time, I had a situation where I had substantially changed the hardware and it wouldn't activate. So I called them up on the toll free number which pops up and explained to them that I had made some changes and they gave me a new code and it got activated.
I believe if you don't do this too often they don't mind. Also, in about 4 or 6 months, it resets and even if major hardware changes are made it just activates via internet. Overall, activation is pretty straightforward and they are lenient towards it.
But I still prefer my corporate version which needs no activation.