Software should be licensed on a per user basis not a per machine basis. If you are not using the software at the same time you should be allowed to install on as many PC's as you like.
Also tying a license to a specific computer in the case of OEM licenses is wholely unfair. There is no justification for it - if i sell my old pc why shouldn't i be allowed to remove XP from it and install on my new PC.
License agreements are far too restrictive - i mean you wouldn't buy cd on the basis that you could only listen to it in your house - if you want to listen in the car or on your MP3 you need to buy another copy. Nor do you get some discount DVD's when you buy your DVD player to be told you can only use those DVD's in that player. Software Licensing is just one big money spinner.