*Vista has a lot of bugs in it so sometimes the main source of connection it would pick is the wireless. disable or turn off the wifi capabilty of the laptop.
*Check if the Vista laptop is getting an IP address from router. Since you've said that there are 4 computer in the network, the network is set up this way. Modem>Router>Switch/hub>PC's. Unless of course you're using industrial grade router as what pekto said.
*If you are using a switch, try other ports or better yet, directly connecting the Vista PC to the router and see if it gets an IP address.
Try ko po to sir, i'll post the results pag natry ko na... tnx po!
