Internet Sharing with ADSL

Check your router settings. Specifically DHCP setting on the router. If the DHCP setup is only allocating one ip address then you can experience only one internet connection through the router at a time. Make sure that DHCP is set to allocate as many ip addresses minimum as you have computer clients for (with two computers, I'd set it to allocate four ip addresses).

If you are not using your router as a DHCP server, then check your manual ip addresses (ip, subnet and gateway) and make sure that they are correct (ip addresses diffeent, but gateway and subnet mask the same). Make sure the gateway points to the router's ip address.

If the above does not help, then you need to provide us with more info: Who is your ISP? Who manufacturers your router? Model of router? And DHCP info.

Your problem seems to be that your ISP has only attributed one IP address. You mention that you have a router, but my guess is that the device is an ADSL modem only, in which case it will only support one connection. If you have two devices (an ADSLmodem such as the Alcatel SpeedTouch Home, and a broadband router such as the SMC Barricade), your problem lies with the configuration of Windows XP, but that's less likely as you're obviously using DHCP (or you couldn't release the address).

Please provide some more details: brand name and type of the "router", name of your ISP, etc. to allow me to give a more specific answer.