Problem with XP Key

I just reformatted a computer that had XP Professional on it and put 2k pro on it (Academic version for our school). And I wanted to put the XP Professional on a little higher end PC for our office. So I jot down the Serial number for XPP and formate it. I just finished installing the base for XPP on the other machine and realize that the product key is too short. (I called MS to verify the key before I formatted) Does anyone know of a way to tell what version of XPP your S/N is for. The computer was donated some time back so I don’t even know who donated it to find out what they used (upgrade, OEM, Retail, etc). I talked to MS and they didn’t get me anywhere so I kinda stuck and don’t want to revert back to 2kp or XPH. But if I can’t figure this out I may have to.
Product Key is 20 digits, dono if that helps.
(I may have forgotten to get the last 5 digits but don’t think I did)

All MS products are 20 digits these days. I doubt anyone can tell you what version of XP you are using just by examining the serial number.
I guess your methodology of transfering operating systems is also very very illegal.

If you have the original cd you can try to match it up here.
http://www.tacktech.com/display.cfm?ttid=226

Unlike Windows 95 & 98, XP only displays the product key that is calculated from the install key. 95 & 98 used to use this product key for system installs but XP does not. The install key can be read from the registry using a special program (google for it) but only if XP is currently installed.

Thanks.