What does the license permit?
The OEM license is the one that says that the software is tied to the newly built, resold box that it was originally installed on. Anecdotally I have read that you can make the case to MS successfully that the old machine no longer works and they will allow the OS to be activated on new hardware.
The OEM version of Windows 7 is the $149 one at TigerDirect. The retail version (which is not tied to one PC alone) is the $300 license.
A bundled version of Windows that came with a Dell or HP, you're most likely out of luck. I'm only talking about a separate product that was purchased by a white box maker or by you.
You may have to call MS's 1-800 number and explain that your old PC was destroyed or the mainboard blew out, and you need to transfer the OS to a direct replacement. I've heard that this is what you tell them.
They may do this.
You probably can't do this with online activation, Unix, because it is probably not a directly permitted operation. You may need to get on the horn.