Are you sure it's authentic...
You should be able to get past this by getting hold of a Service Pack 2 Installation CD
, to install SP2 (that is what you're trying to do...right?).
Microsoft did send SP2 updates on CDs through mail for free some time ago...got mine by doing just that. Not sure if they send them for free anymore. But you should be able to find one on eBay, or maybe on a P2P somewhere like bittorrent.