Here is a general guide I wrote. For Vista, however, I think you will have to go to the sharing tab instead of the advanced tab when you are browsing the properties of a connection...
1. Get a CAT5 or CAT6 crossover cable. The cable that came with the PS3 is a normal Ethernet cable, therefore it cannot be used for this operation.
2. Make sure you have two sources for network communication. For example, you can have 2 ethernet ports on your computer, or you can have a wireless card and a Ethernet port etc. It helps to draw out a connection blueprint such as this:
DSL Modem > CAT5 cable > Network Card 1 > Computer > Network Card 2 > CAT5 Crossover cable > PS3
You can buy a USB Ethernet adapter for the second network connection on the laptop.
3. Connect your computer to the modem and verify that you have an Internet connection. Then, click on Start, go to settings, control panel. Double click on Network Connections, and identify the connection to your modem (Network card 1).
4. Right click on the modem connection, and choose properties. Next, click on the advanced tab (Sharing tab for Vista users), and check the “allow other users to connect…” checkbox. After the checkbox is checked, click on the “Home networking connection” drop down box and select the name of the second network connection. Click OK and exit the properties window.
5. Finally, connect the CAT5 crossover cable from the second network adapter of the computer to the PS3, and voila, your Internet connection is now routed to your PS3. Please be aware that your computer must be on in order for your PS3 to stay online. In the long run, a router may be a much better device to use than your computer, since it consumes only a fraction of the electricity than your computer does.