There are some bad GOTY CD keys out there!

If you've purchased OFP's Game Of The Year (GOTY) Edition and cannot get OFP to accept the CD key printed in OFP's Field Manual, the CD key may be bad. Codemasters has stated that some of the GOTY manuals had incorrect CD keys printed in them.

If you think that this is the problem that you're encountering, your best bet is to contact Codemasters and tell them so.