i think with the 3 pc's one Microsoft are supposed to be bringing out a 'family licence version' i.e for 3 computers in the same household, although how they would enforce it i don't know. rumour has it it will be about $149 in the usa. but more expensive over here of course.
if you put the hard drive from one computer into another it may not work as the hard drive will be set up with all the hardware drivers for the original one. you could try and download them from the makers web-site.
supposedly when you register/activate a copy of windows it is linked to that one computer...you could try and do a fresh install on the b one but it may not activate windows properly when you try.

although of course if you put the a hard drive in to computer b, and then loaded windows straight onto that, it should work ok. although there may be some problems with some hardware drivers, which can be solved as previously mentioned.[or indeed if you the original drivers disc for that computer]

Thats interesting to hear. I'm wondering if there's a difference in the way that the HAL is working on W7....that sounds like a job for this afternoon!