If your two wireless laptops are working properly, then you've not setup something properly on the computer.
Control Panel -> Network Connections.
If you're on Windows Vista, You'll need:
Control Panel -> Network and Sharing Center -> Manage Network Connections
Right click on the wired connection (it'll normally be called Local Area Connection) or similar, and hit properties.
Scroll down to "Internet Protocol (TCP/IP)" and click it, then click Properties. (Vista will probably make a distinction on version 4 vs. version 6, we're looking for version 4 here).
Compare the settings at the TCP/IP Properties screen on your laptop versus that of the PC that doesn't work.
The only thing that should be different is the IP address, assuming you're manually setting addresses. Chances are you have a typo that requires fixing here.
If that doesn't work, right click on the Local Area Connection and click "Repair". This will perform all the basic maintenance you need on the connection (ip release, ip renew, flush dns, etc) and should help in getting it working.