Hijacked network? Help!

I started getting this error message this morning:

"Windows -- System error There is an IP address conflict with another system on the network."

Now, AFAIK, there IS no other system on my network. Is there a way to view the computers connected to my network, or check to see if I've got a bug?

I am running Nortin Internet Security and Norton Antivirus. I've also run Ad-Aware and Spybot Search and Destroy. None of them have detected anthing...

 

Is this a wired or wireless network?
If it's wired, set the ip address to automatically configure.
If it's wireless, have you got any security on the network?