[bedoig]

What program are you using to tether? Some aren't good about cleaning up after themselves and leave the phone set to a static IP - typically 192.168.1.1 which conflicts with your router which is probably trying to use the same address.

Depends somewhat on what version of WM you're running, but in general:

Start --> Settings --> Connections --> Wifi --> Wireless Networks --> Network Adpaters tab

Click on your wifi adapter (802.11...) and select the radio button for "Use server-assigned IP address"