hi So far i have added both computers to the same workgroup (one is a windows xp and one is a vista homepremium) however in the windows xp computer you can see both computers in the network. In the vista computer it can see itself and the router. Does the router need to be added to the workgroup?
try disabling the vista firewall, also what antivirus do u have, if its norton then thats your problem i had the same issue
