Sharing mac connection with pc, slight hiccup

Hi guys, I'm trying to share my Ibook's airport wireless connection with my desktop PC (no wireless on it, runningon vista). So I connected both computers via ethernet and they can see eachother. So, thereafter I went on the mac and did network -> internet -> share via ethernet. I thought this would be the end of but not so. My PC cannot connect to the internet, it just times out. While on the PC, I tried clicking on the network icon, but it prompts for a username/password. I've tried all sorts of combinations, but it never seems to work. Am I missing anything?

Thanks