The AVG free edition found that there's a trojan PSW.Briss.A virus in the file C:\System Volume Information\_restore{EE91A5DC-153F-4FFA-BE64-A37F82576208}\RPR4\A0017991.exe The problem with AVG is, that the AVG Shield finds the virus, but the actual program doesn't. I've also tried multiple anti-trojan software (TDS, Digital, Pest Patrol...), to no avail, none finds it. The problem is, I think, that I can't even open the System Volume Information -folder, neither can I delete it. Please, help me with getting rid of this virus. Thanks.

From the information you provided, the tojan file is in "system restore". You need to turn off "system restore" and restart the computer. Then run AVG to make sure the system is clean. You can turn "system restore" back on if you want to continue to use it. Files in "system restore" are protected and no antivirus program can clean/delete them. Take care and all the best!

Ok, thanks, but how do I turn off "system restore" ?

Ok I think I found out how to do that. Still, AVG didn't find the virus, neither did any of the other progs. Interestingly, when I hover my mouse pointer on top of that "System Volume Information" -folder it says "folder is empty". Still, there seems to be a virus in it. It might be this same thing that is fooling all these anti-virus software. How to unravel this one, anyone..?

Go to http://www.computing.net/security/wwwboard/forum/10647.html The registry removal instructions for this password stealing trojan is listed there.

I used TuneUP Utilities' Registry Editor to go to the path informed in the link that you provided, but there's no "systray" in that reg directory ("Run").