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1. Click Start, and then click My Network Places.
2. Under Network Tasks, click View Network Connections. (Or, right-click My Network Places on the desktop, and then click Properties.)
3. Right-click the connection that you use for the Internet, and then click Properties.
4. Click the Advanced tab, and then click Settings.

Note If the Settings button is unavailable, ICF is not enabled on this connection, and you do not have to open any ports (because they are all already open).
5. Click Add to open a new port.
6. In the Description box, type a friendly name. For example, type File Sharing : Port 445.
7. In the Name or IP address of the computer hosting this service on your network box, type 127.0.0.1.

Note You can specify the IP address of an internal computer. But you typically will use 127.0.0.1.
8. In the External port and Internal port boxes, type the port number. Generally, this number is the same.
9. Click either TCP or UDP, and then click OK.
10. Repeat steps 1 through 9 for each port that you want to open.

For vista start, network and everythings the same

http://support.microsoft.com/kb/308127

This just opens a port on your computer - It requires a port opened in your router aswell.