You can connect either computer at different times by connecting your dial-up modem to it and connecting via any phone line.
You can also have both computers connected simultaneously by using a feature in Windows called
"Internet Connection Sharing". Connect to the internet with your desktop PC, link the two computers directly with a
crossover ethernet cable and enable
"Internet Connection Sharing" on the Modem connection in the PC. There are instructions for enabling
"Internet Connection Sharing" in XP or Vista
<u>HERE</u> - ignore the references to an Ad Hoc Wireless Network, you'll just use a crossover ethernet cable, unless you actually want to set up a wireless link between computers.
Although
"Internet Connection Sharing" will let you have both computers connected simultaneously, dial-up really is too slow to do anything useful with more than one computer at a time. Also, of course, the desktop PC must be running and online for the Laptop to use its "shared" internet connection.