I run a shop remember???
Here's the drill>
OEM means I would normally install this on a system I built and sold. It has all the features of the retail version. The difference is in the tech support. Retail you get it, OEM you don't. Or you're not supposed to. But I've had customers come back and tell me MS has been pretty cool about helping out. As it's supposed to work, tech support is MY responcebilty.
Now as for more than one install here is where the rumore mills get churning.
One disc, One product key code, One machine.
If you have problems that can't be resolved by using xp's built in system repair tools then you are allowed to reinstall the system. The limit was 6 at one time but a friend of mine managed 8 before MS put its foot down and he had to call tech support. One more time he has to reinstall and he has to buy a new cd or just a COA/key code.
As for putting the same key code on more than one machine that's out and out piracy. The first machine that phones in with the key code is the one MS acknowlages as the legit user. All others are pirate copies and are treated as such.
UNLESS you get the XP Pro Corp version with multi license option. Costly as the smallest version is 5. If you get the unlimited version you better reach R E A L deep in your pocket.