Just use the windows 7 dvd and do a clean install. Specify the version(home premium I presume), but do not activate.
That gives you a valid windows 7 pc for 30 days. Now when in windows, insert your dvd again, and open it, asking for an upgrade install. When finished, your update activation code will work. You can then delete the first install which will have been saved in windows.old.
This procedure is legitimate, and is needed when upgrading from a 32 bit os to a 64 bit os.