I also have the latest version of Avast free - trying to install on AMD64 3700, WinXP Home SP3, (all updates are up-to-date) - with FF 3.6.3. Version 4.8 ran faultlessly. Version 5 seems to install OK, but
on the SECOND reboot, it appears disabled in the system tray (triangular ! icon) and will NOT allow the bootup to finish. (20 minutes + and can't click on anything)
ONLY recourse is to Hard-reset and uninstall in SAFE mode
I have tried "Repair" - which is available in the CP Uninstall procedure - no change
I have tried loading Avast AFTER system files have loaded - no change
I have confirmed DIRECT connection (rather than AUTO) to the internet (I don't use a proxy) - no change
I AM running ZoneAlarm 8.0.298.000 (which is I think the latest) concurrently (which was fine with Avast 4.

I am at a loss to know what I am doing wrong.
I really like Avast - and have installed it for many aquaintances of mine - all of which have been grateful for the protection it affords.
Has anyone else had this "strangeness" with the latest version?