<<...and some users suggested me to run ComboFix (I did not know it was such a powerful program, and they seemed reasonably experienced). I ran it, and it actually fixed my problem for a while. But now I'm back where I started>>
Since it seems to me that you are convinced that you have a malware problem...I suggest that you do the following.
Follow the steps in this guide
and post a topic, including the requested logs as well as the combofix log
in the forum linked to in the Preparation Guide (my link).
Once you have done that, I will close this topic and all your energies should be directed to your new malware log topic.
Edited by hamluis, 11 March 2012 - 10:16 AM.