|First, check the connection speed on the network icon in your task bar. If you don't have that icon, go into the property sheet for that NIC and click the box that says to show it in the systray. Chances are your switch has 100 Mbps LAN ports. If your NIC shows it's connected at 100 Mbps, then at least you know you're connecting at the proper rate.|
That doesn't preclude an issue with the NIC, but it does rule out one issue in particular.
If you feel it may be the NIC try another NIC. They're pretty cheap these days so buying one shouldn't break your budget if you don't already have a spare you can use to troubleshoot with. If you try another NIC and get the same result, then it's not likely the NIC is the problem. In that case, try a different port on the router.
Since you have a WRT54GS, you could run a 3'd party firmware which may also fix the issue. Or, conversely, if you already are running a 3'd party firmware, that may be the problem. Anyhow, if you're not, and are interested in looking at some 3'd party firmware, here's a link to my favorite 3'd party firmware, Tomato. There are 7 or 8 more 3'd party firmwares listed at the bottom of that page in case you'd like to check a different one out. A lot of people use, and like, the DD-WRT