Is the problem machine using a 64 Bit Op system? If so there is a good chance that your router will need a firmware update to work with Win 7 64 bit. There have been a few reported problems with your model of router and 64bit Op systems. The latest firmware shows some fixes for 64 bit that should help. It might also help to look around for some tested Win 7 drivers for the problem machine.
Linksys by Cisco - WRT54G Support