I just downloaded Avast 7 to 2 computers, it shut both of them down to the point that one won’t statr and the other won’t start in Safe mode. As I amnow on my 3rd computr with Avast turned off, it appears that if there is a virus it came with Avast 7 and the Google chrome that it includes!
What antivirus did You use before installing avast ?
Did You uninstall it before installing avast ?
Did You use a removal tool to clear any leftover files that may conflict ?