My network has been penetrated by a rogue PC!

I have a serious problem: I think that a hacker has broken into my network, and I am troubled. Since I created this peer-to-peer network, 3 years ago, I have only had 3 PCs on it running the following OPSYSs.

1. pc1 = Hhr (Win98 R2)
2. pc2 = Main ( WinXP Pro ServPak2)
3. pc3 = Dell (Win 98 R2)

I suddenly find (it may have been there for awhile) a fourth foreign PC called: “Rongee-pc”
I am certain that it has been put there by a hacker.
I never had a fourth PC on the network, let alone one named “Rongee-pc”!
It is inaccessible to me, whereasthe others are completely accessible.

I have been using the free AVG anti-virus.
I have Windows Firewall “On” with a list of exceptions (none are Rongee-pc!)
I have a Linksys Broadband Router Hub, and a Cable Modem for Internet connection

I am dreadfully afraid of what this hacker PC might do.
Do you have any ideas?

 

DO u have logs for the pc's?

Well if u have them attach them here and post.
Lemme try to analyse.

 

Hell this is a Jan post i am late.