Lost internet connection: can't contact DHCP server

Suddenly while browsing, about two weeks ago, my desktop computer lost its internet connectivity. It has a built-in networking thingy. It is connected to a router, as is another desktop and a laptop (by cables), both of which access the internet fine, so there should be nothing wrong with the modem or router. The green led comes on when I plug the cable in. When I run ipconfig /renew, I get "unable to contact your DHCP server ". I can't even access the router (192.168.1.1). I get a 169.254.100.30 address.

My board:
MSI Mainboard S-939 nForce 4 Ultra ATX PCI-E Audio GbLAN 1394 SATA.

I've tried:
* Plugging the cable into other ports as well as straight into the modem
* Changing cable (using the ones the working computers use)
* Restarted computer and modem several times
* Inserted another network card. Two, actually. I took them from older computers I had. Coincidentally, they were the same card: a realtek RTL8139c. I installed the drivers from realtek's website, but this does not seem to work. I get "Media state ... : Media disconnected" if I run ipconfig with the cable plugged into either of these. Also, the led doesn't light up. Neither do these devices show up in the device manager. Maybe I've done something wrong, but what more should I have to do than plug them in and install drivers?
* Reinstalling Windows XP (I've installed the drivers for the built-in networking thingy). The old XP installation had no firewall or virus programs to interfere with the connection. The new installation only has Windows's built-in firewall running.
* Run lspfix (which found no problems) and WinsockxpFix
* Run netsh int ip reset

Is there any more info I need to supply?

Any help is appreciated, even if you can just give me advice on where else to ask or read.

It should be added that my computer in the last few months has been fubar. One day it suddenly had problems recognising some of my drives, either not finding them at all (at the startup screen) or giving them garbled names like S-FOO-14 or SAMSUNG-GFHJ`````````````````3G`` (the latter wasn't precisely like this, but it had nonsensical letter combinations and lots of backticks) and then not finding them in Windows. Sometimes I couldn't even reach the BIOS. The only way I could solve it was to disconnect two of my harddrives (I could still access them as external drives via USB though) and live with the fact that I couldn't use my CD drive. The only special things I did right before this happened - if you're interested - was to install the newest version of Daemon-Tools and unplug my iPod while the computer was starting up.

That's a more serious problem that I'd like solved as well, but without internet my computer is pretty much useless, so let's solve this first.

 

I get replies when I ping 127.0.0.1 and 169.254.100.30 ( "autoconfiguration ip address "), but I get "destination host unreachable" when I ping 192.168.1.1 (default gateway on the laptop, which is the same as the ip of my router; my desktop has no default gateway, just an empty string) and likewise for 192.168.1.3 (what my router should give me) and my "external" ip 213.144.....

 

The symptoms are classic for faulty hardware, but you've tried all the obvious hardware swap outs. However, there is one you might not have tried - different PCI slot on the motherboard. Try the network card in a different slot.

The reason I suggest this is the reported lack of LEDs lighting on the network card. That makes me wonder if there is a power supply problem to the card itself - that is, a faulty PCI slot on the motherboard.

 

Well, I had three empty PCI slots and I put the two network cards in the lower two. I guess I could try the third.

If it's a hardware fault, what is it that's probably broken? The motherboard? What would it cost to repair (I wouldn't know how to do this myself)? The guarantee for the computer is gone.

 

It's very difficult to say what is wrong with your PC without sitting at it. It could be something small that we have all over-looked, but the issues you are having with devices needing to be disconnected does suggest there is something wrong with your motherboard.

How old is you PC?

If it is a few years old, I would not just change the motherboard. You'll need new PSU, CPU and RAM to match a new motherboard. By the time you get them all fitted and working with your original kit, it will probably cost about the same as buying a new cheap PC.

However, reading through your post again I've just spotted something. You're on board NIC uses the same drivers as the replacement cards you are using. This could be a resource/driver clash. Before doing anything else, I would try using a non-realtek network card. Unfortunately many cheap network cards are actually rebranded realtek cards so you might have to buy a card from a well known specialist network card manufacture like 3Com, or Intel who will use their own chips rather than resell realtek chipsets. You can pick up cheap second hand 3com cards very cheap (independent computer shops often have a bin of them), so it might be worth getting one of those to test the system.

Another thing to check is if you can disable the on-board NIC in the BIOS.