Ravenheart: as with most data the client sends to the server, the server has no means to verify that the information is correct... You wouldn't need to reinstall Windows at all - you'd have three easier possibilities:
- Change the Windows installation date in the registry (easier than spoofing an IP actually).
- Use a modified client that sends a different installation date to the server. As said, the server has to means to prove that what you tell it is true.
- If the client is so strongly protected that modifying it is difficult (which I hope will be the case), there's still the option of making a proxy server that does nothing but forward all traffic to the real server, but modifies the installation date if it is encountered. This is a sort of man-in-the-middle attack and is rather difficult to detect...
You can argue that the latter two points take work, but all it takes is 1 dedicated cracker really. Just look at sa-mp.
(And the above three points don't just apply to the Windows installation date but to any identifying information that can't be verified by the server. The forum account idea sounds better to me. Creating a new account is more hassle than setting a new installation date)