Remaoval of Viruses and Trojans in systems in Lan

In our Office we have 25(Win XP) client machines and one server(Win 2003).
One system got affected with virus(Win32:Brontok and trojans) and all the other machines get affected including server as the machines were in LAN.
We run Trendmicro antivrus with daily updates. As the antivirus was unable to remove viruses. We again installed Avast 4.7 Professional, updated and scanned, but there was no result. We formatted all the systems excluding sever. All went right upto ten to twelve months, But now all machines are becoming slow. Viruses Win32:Brontok and trojans pop up window message showing when scanned thorugh Avast 4.7 Professional in few machines
We dont want to format and reinstall the server
Is there any complete solution for this scenario, Trojans and viruses should be removed in all systems without affecting the valuable data.

Thanks in adavance for neccessary help.

 

Welcome to WindowsBBS svshivanand

Brontok is a very nasty infection, and difficult to remove. There is a tool for it however. I'm checking to see if it's compatible with Server 2003.

Do you happen to have an image of the server, or can you create one just prior to running any tool(s) in the event of an unrecoverable problem? You should have a good backup of all valuable data.

Please read this topic and post a main.txt log from Deckards System Scanner as outlined (you need to first install HijackThis, do a scan and save the log ......... we won't need that log though).