Do this while logged into your main account (not guest)
1. Start menu->Settings->Network Connections
2. select your connection (should be Local area connection, if you are unsure print screen the window that has popped up and post it here)
3. Right click local area connection and select Properties
4. Select the advanced tab
5. Tick the box "Allow other network users to connect to through this computers internet connection"
6. Click ok and possibly reboot (I'm not sure)
Give it a go, if it doesnt work get back to me