Both Desktop&Wifi-laptop – not seeing Internet

Helping a friend. Comcast modem. Linksys WRT545G router. XP Pro.

With Desktop only (to modem): works fine, can use internet.

Introduce the router (modem to router Wan, one of ports back to the Desktop RJ45 ethernet).

Now neither one sees the Internet. Both say they are fully connected (can connect a laptop with the security password — actually can use itouch too, same result). Browsers report ‘no can see home page’ etc).

I must be missing something. (It’s obvious I’m not a network guy. )
Someone else tried, and “tried things” before I got involved. So I suspect settings off.

I have searched Google, missed an answer so far.

Thanks (my friend’s daughter needs the wifi for homework – kinda anxious)

Bill