What you would need is a proxy server. You can get a free proxy server at www.analogx.com.
You would then have your main computer connect to the internet (if I am not mistaken, you could automate this, but it isn't very reliable). You would install the proxy server on the main machine and run it.
Then on your other computer(s), you would just set up the proxy connection in your internet browser by following the instructions included in the analogx proxy software download.
I recently bought a brand new laptop. I keep the laptop in my attic where my room is and put my old desktop down 2 stories on the main floor of my house. I was wondering if I could possibly connect to my ISP with both of my computers at the same time. How would I go about that? Also, is there a way I can use my wireless router to connect both computers wirelessly to my dial-up connection? Please give me insight to these questions. Thanks!