Thank you DavidR

OK let’s proceed with order with the information needed

[]OS: Windows 98SE, not up to date at the files detection time.
(but now it is!!)[
]no e-mail client used
(I manage my e-mail account directly by the site)[]Avast version: 4.1 Home edition[]VPS file: unfortunately I don’t remember which it was at the time of detection. But not older than one month ago version[]virus name: Win32:Trojan-gen.{UPX!} (this is what I read in the Avast’s report. The same name for both two files[]files names and their original position: []C:_wmssys.exe[]C:\windows\system\msconfd.exe[*]actions taken: I moved both files in the “virus chest” like suggested by Avast, and compiled the “Virus incident report” on Avast’s site

Moreover, when I noticed my system wasn’t able to shut down (before Avast’s scanning, I checked all active processes with the MS Process viewer, but found out nothing! Is it possible any malicious process to run not in the user space, like a driver or something like this?

Well, that’s all. I hope this information to be enough. if not please let me know.

Thank you
ping1it