Whether you buy OEM or Retail version of Win 7, it is still one license so you can only install it one computer. Each will only have one product activiation key which can only be used on PC at a time.
OEM version requires you to contact Microsoft for permission to "release" the key so that you can perform a reinstallation of Win 7 either on the same PC or a different PC.
The Retail version allows you to re-install as many times as you like on either the same or a different PC.
If you install Win 7 on a different PC while the activation key is still "active" on the original PC and you attempt to use the same activation key, then you will likely get an error message stating the key is already in use. You will be able to use Win 7 for 30 days without an activation key. Based on what I've read, I can be extended upto another 90 days, but after that...
I am currently using a buddy's copy of Win 7 to evaluate whether or not I want to upgrade from Win XP.