Help! Virus Always Here !!!!

hi avast, i have a compaq desktop which makes me feel headache over a year.

since i tried to re-installed the computer using recovery disk or windows xp directly, from which, i deleted the whole partition and re-formated the harddisk before installation, after the installation was completed, i installed the avast immediately and performed scanning from the command screen after re-boot, there was nothing wrong at all, however, i found that once i connected the internet cable to the computer, avast immediately prompted that my computer was infected by MSNGR32.COM which is stored at c:\windows\system32, so i deleted it, after that, i still receive another 2 or 3 prompts, one is saying computer found c:\1.vbs VBS:Small [trJ], so i kept deleting whatever reported.

The question here is that I really don’t understand why the computer seems only infected after i connect the internet cable with the computer, it seems really strange to me, is there anything related to the ISP model or ISP ? or there is a virus hidding somewhere in my computer, pls help.

What version of Windows do you have?

Do you have a firewall?

http://www.bleepingcomputer.com/startups/MSNGR32.com-18835.html