Need a bit more information on this one.
Are you using Microsoft Internet Connection Sharing with an ADSL modem usb or internal PCI card in either of the PCs? If so, which PC is connected to the Internet?
Or are you running an ADSL router/switch?
Are you running any kind of Firewall, such as Zonealarm or Norton Firewall?
Check that your Tools/Internet Options/Privacy settings are the same.
Are all the PCs on your network running the same operating system?
RobAnt

I'm a Friend of XH558