Avast! update clashes with ThreatFire
Avast! has reported that in Avast! version 5.1.889, which has been available since mid-January, the Behaviour Shield module is now partially enabled – this module was previously disabled. If another behaviour recognition system is already active on the same computer, this may cause programs to crash or refuse to start. On the Avast forum, users have reported problems particularly with Avast! and ThreatFire on 64-bit Windows 7 systems.
To solve the problem, Avast! has recommended uninstalling the additional behaviour recognition software (ThreatFire) or removing Avast's Behaviour Shield module by performing a custom installation.