AvastemUpdate.exe Trojan Downloader???

I just updated avast! to 9.0.2011 and after a reboot I received an alert that a Trojan Downloader was blocked: AvastEmUpdate.exe, found in avast folder itself. This happened for the following two reboots. At the third, it didn’t happen.
What could be happening?

windows xp sp3

already posted in several topic all over the forum