I have a Windows 7 system with wired shared Internet connection, and wireless ad-hoc network with 1 Vista PC. Randomly my wireless network is marked as 'No Internet Access' and ICS is then unfunctional. Network icon in the tray displays exclamation sign, but no error message. Rebooting sometimes fixes the problem, sometimes not. What can cause this?

Exclamation sign in device manager>network adapter? Btw for ICS to work you need to have the main comp (let's call it server) to be on, then the other pc (let's call i client) then could access the net. Also ensure there's no third party firewall or else you would need to configure it

No, adapter seems to be OK, it's connection icon in system tray which shows it. There's no third party firewall. When my installation works, client computer has Internet access and 'Access type' is set to 'Internet Access' on both machines. The problem is that sometimes my wireless connection works fine, but Internet access capability is lost. In Network & Sharing Center it's marked as 'No internet access' on host, and 'Local network' on guest.

You may need to check the routing table when the issue occurs. I suspect that the default route of the ICS host computer is changed to the address of the other computer with AD-hoc connection. The following articles can help us to understand Default Gateway in system. Default Gateway Configuration for Multihomed Computers The IP routing table: TCP/IP To check the routing table, please enter the following command in cmd. route print Please check if the following route exists at that time. Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 [the Gateway of Internet connection] [IP address of the Internet connection] If not, please use the following command to add this route. route add d 0.0.0.0 mask 255.0.0.0 [the Gateway of Internet connection] if IP address of the Internet connection] The following article has introduced how to add a static IP route Add a static IP routeArthur Xie - MSFT