I'm having some trouble with my main PC today. Everything was working fine last night. I walked in this morning to find that my LAN connection wasn't working properly. I left the computer running overnight and I am the only person that uses it. The notification icon in the system tray says that my ethernet connection is continually disconnecting and reconnecting every 2 seconds or so.

I tried connecting the computer to 4 different Linksys WRT54G's with different firmware, 1 Cisco router, and directly to my cable modem. No luck. When I unplug the ethernet cable from the back of the computer it stops the rapid cycle.

It sometimes switches on and off so rapidly that a Web page will load without timing out.

Here's the specs on my PC, if they help any:
Windows XP Professional
Abit KN8 SLI Motherboard
AMD Athlon 64 X2 3800+
1GB (512MB x 2) Corsair XMS
250GB Western Digital
Nvidia GeForce 6600 x 2 (SLI)

Edit: Tried it out in Windows Vista RC1...It gives a "limited or no connectivity" warning, then disconnects. It doesn't try to reconnect again. Seems to be a hardware problem.

You have isolated the problem to your pc. Check to see if there are any new drivers for your pc. Looks like you may need to add a new nic if out of warranty. I am assuming this is a wired connection and not wireless

My friend had the same problem he changed the network speed from 100 mbps to 10 mbps full duplex, the problem disappeared. He got a new lan card couple of days later.

Spyware
Virus
Set NIC to 100/full duplex.

Possible virus or even a DoS on your computer.