You could use your existing equipment by using either a Proxy Server (software) or ICS - Internet Connection Sharing, which is effectively, Microsoft's version of a proxy server. The disadvantage to this, is the computer connected to the modem must be on for any other computer on your wireless LAN to be able to access the Internet.
You could change your modem to a type that supports connection to a router (RJ45). This option would mean the modem feeds directly into the router, and any computer that can access the router, can also access the internet independently of anything else.
You could change the router in a number of different ways.
The first would be to buy a router that supported a USB connection for your modem, and you could access the net as above.
Or, you could buy an ADSL wireless router that would combine all those functions.
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