My PC's Ethernet seems to be screwed up. Some time last week, after a reboot, the Ethernet on the motherboard seems to have just quit (in both Linux and Windows). It may be the router (a Linksys WRT54G with the latest firmware), but it is allowing the laptop to connect just fine. It doesn't recognize that there's another computer on the other end if I connect it to the laptop, but that may be because it is expecting the Internet, as the laptop acts the same way in that situation. Any ideas?

well, to connect two computers without a router/switch, you need a crossover cable, so that is why the computers didn't see each other that way. as for the router, try a different port on the router and see if your laptop can use the port the other computer uses.

I know of many Ethernet chipsets (mainly more modern ones) that claim to detect if it is a crossover cable or not and will automatically compensate (so that you DON'T need a crossover cable to connect two computers together) - I was able to successfully connect my PC and laptop directly together using both a crossover cable and a regular Ethernet cable.

HOWEVER, by default neither Linux nor Windows has a running DHCP server, meaning that when you directly connect them they both start looking for a DHCP server, and will of course fail to find one. You have two solutions to that (both fairly easy). One is to just set them both up as static IPs (give one of them, say, 192.168.2.1 and the other 192.168.2.2). Or, if the laptop is connecting via WiFi, just tell windows to share the wireless connection - which will then cause windows to act as a DHCP server on the Ethernet connection. Once you did one of those methods then try connecting the laptop and desktop directly together and seeing if it works.

actually, you don't need to do ICS, if one machine is set to 192.168.0.1 in Windows, it acts as DHCP (tested between two laptops with no wifi at that time).

Really? I didn't know that... I knew that Windows will force your other network connections to 192.168.0.1 if ICS was enabled, but thanks for that tip - I was wondering how to enable the DHCP server without ICS .

It doesn't matter which ports on the router is used, the laptop works in all and the PC doesn't work in any. What is the difference between a crossover cable and a regular ethernet cable? As for Wifi, the laptop isn't able to detect the wireless signal even though there's 3 feet between the two. I've tried to remove all the security from the wireless router, but they still don't find each other. I did check frequencies and channels, but it was still lost. And how do I set up a static IP?

http://en.wikipedia.org/wiki/Ethernet_crossover_cable

To change it to a static IP go to control panel->network connections, right click on the connection you want to set as static->properties. In the "connection uses following whatever" box, highlight the "Internet Protocol TCP/IP" and then hit the "Properties" button. You will then see the radio buttons for "Obtain address automatically" - instead, change it to "Use the following...", then enter 192.168.0.1 for IP, and 255.255.255.0 for subnet mask (should fill this in for you, though) - the rest can be left blank (since it won't have internet access anyway)

I have some updates on what's going on. I believe that the problem has been narrowed down to a Windows problem (even though the ports didn't work in Linux either). I put in an old Ethernet card we had laying around and Windows still says that there's limited connectivity, but Linux (Fedora 7) uses it just fine. The motherboard ports are still iffy in Linux (I'll try them tomorrow if I have time), but I think they work now. I'm tempted to throw all my data onto my external HDD and do a reinstall, but I don't think that will help too much at all. Any ideas with this new information?

In windows you did make sure you turned off ICS and told it to automatically acquire a network address, correct?