(Vik - hope Ok if I move my U3 posts to this forum from general)
So far the Avast! U3 (for USB stick that one moves between PC hosts) has been well tolerated by other security suites on the host PC - no problem with Avast, GriSoft, Avira, Norton, and MicroTrend. 
However the other day I tried to mount my USB drive on a computer with AOL’s “standard” security package. ??? Avast! U3 starts fine - scans host, checks for updates, etc … but at the point when it flashes the “installing” message the PC literally bogs down and something locks in a deadly embrace. I ended up power-cycling the system as nothing (even Task Manager) can get enough cycles to run. I tried one more time just to be sure - same result. I did find if I hit the “X”/Close box of the Avast U3 popup before it gets far I can successfully mount the USB drive.
Actually, feature request - to it would be nice to add a checkbox to the “trusted hosts” list which prevents Avast U3 from installing itself - I don’t mind the sanity checks during startup, but if it is a trusted host (ie: one I manage and has another Anti-Virus installed) I don’t see the value of having yet-another TSR/background task running to check files for Viruses.