Win 8 RC and preview versions will all expire on Jan the 15th. But if you've got any XP/Win7 serial (working one) lying around, just take the opportunity to upgrade to Win8 Pro for only £24.99 (just convert to your currency) or £49 for a DVD. Not bad.
Anyway, anyone running Mac OS X 10.6.8 before installing Win 8 can tell me if there's any difference in speed?