I disabled the Windows Search the day when I started this thread. The problem were there, but a bit differently. But all was written in my first post.

Now, I tried again.

I set the File System Shield to Repair, if it fails then Move to Chest and if it fails then Block (no action).
I downloaded eicar.com via SSL to the Desktop.
avast! couldn’t move to chest it, it only blocked it.
The eicar.com was there for about a second, and then a .tmp appeared.
I scanned (from context menu) the file, and tried to move it to chest or delete it, avast! gave me the following error: There aren’t any space left on the disc (sorry for the bad translation, I translated it from Hungarian ;D ).

To sum up, there’s something that deletes the origin file, prevents avast! from “hurt” it, and creates a .tmp file.

I don’t know which process, because avast! writes the browser as the accessing process.

Please help me, I don’t want an undeletable trojan or virus or something worse!