You have to setup a network. Go to Control Panel> Network Setup Wizard>
Click once> Cick Next two times > Select First Option
Then select the bluetooth DUN modem (If u use USB Modem then select USB Modem)
When the setting will be configured then u have to restart ur computer.
U should be able to share internet connection.
After restarting u can manually set up ur and other PC's IP addresses.
Say, ur IP is;
Subnet MASK: 255.255.255.0
Default Gateway: 0.0.0.0
For the other PC (Which will share the internet);
SUbnet Mask: 255.255.255.0
Default Gateway: 192.168.110.2 (it should be ur IP).