Basically, you will not be able to re-activate your OEM software on a replacement PC. OEM is a one-shot deal. You should still be able to re-install it on the PC that it was originally installed on.
Saab .... it goes further than that, much further.
Microsoft make it clear that OEM software has to be pre-installed. Therefore unless the poster works for an IT company, he/she will be acting illegally from the word go simply by virtue of installing the software themselves. It is also against Microsoft license terms for resellers to market OEM as independently purchasable. OEM must be bundled, always !!! Therefore it is probably also in breach of license terms for a private individual or company to buy OEM and install it on any old PC.
There's nothing stopping you from re-installing (subject to licensing) OEM software, but the first install must be a pre-install by the vendor.
Other than that, as Saab says .... OEM lives and dies with the computer. And with Microsoft's current fancy for mandatory activation, I very much doubt you'll be able to find a way around it.
There is a reason OEM is so cheap .... it's because of the very, very strict licensing terms.
One side benefit of FPP is that you get to install it on PC and laptop for use by the licensee (no simultaneous usage though !).
Ref:
Microsoft OEM License Terms
http://www.microsoft.com/oem/sblicense/default.mspx
Clauses 4,5,6 are particularly relevant. Note also clause 8.