Legally you can't do it - you must have a seperate licence for each machine, and the "sticky label" licences are only meant to be used for the box woth which they were bought (as they are usually very discounted OEM licences). But you weren't asking that ;-)
I have a sneaking suspicion that when you install the second computer and register the key then the Microsoft server will recognise that it was recently used on a PC with a different "hardware configuration" and make you ring Microsoft for a manual registration key.
For XP that is no problem - they just ask you to confirm you are only using this on one machine and give you some numebrs to punch in. For Vista I have a sneaky feeling that updates get knobbled on one of the machines.
Google will no doubt be your best friend.