|CurtR, I believe the OP has one of those USB modems that utilize cellphone network to connect to his ISP wirelessly.|
In that case sharing this internet connection is not as simple as the usual cable/DSL type service.
And to depakjan, who asks "cant i do this with some 20$ wireless router or something like that"
short answer No.
Your PC needs to be equipped with a NIC ( Probably has at least one) and you need to be able to create a bridge between the USB modem connection and the NIC in order to share the internet connection. You may need some kind of proxy software although Win7 may have the necessary inbuilt in ICS.
You then need some kind of wireless router or accesspoint connected to the NIC. In my experience the cost of routers & accesspoints is similar and you aren't going to get one for $20.
I'm not even certain you will easily set up such a network.
Goin' Fishin' (Some day)