Problem solved!! ;D

Your questions put me on right track - in fact, there was a trial version of Trend Micro Titanium Internet Security preinstalled with the system. Nasty stuff, it also conflicted with AVG (I couldn’t even finish installing it) and Microsoft Security Essential. Wouldn’t have thought of that, because I have it on my Asus, too, and it gave me no problems whatsoever. Anyways, I removed it, installed Avast 6 and everything works like wonder.

Thank you very much!