OK, now it's my turn to have trouble...

I've run Avast Pro 4.x for many years with no problems at all.
Updated to 5.0.377 when it was released. No problems... Just installed over 4.8
Tried to install the new version 393 and then the problems started.
PC froze on startup - Avast icon spins two times and then PC freezes, even the mouse goes dead..
Uninstalled in safe mode and tried a new fresh install - same problems...
Reboot in safe mode and unticked the Behaviour Shield in Add/Remove programs.
PC starts just fine...
Tried one more time to activate the Behaviour Shield, and PC freezes after reboot.
Disabled the Behaviour Shield one more time, and everything is back to normal.
...so as a summary: With 377 everything runs just fine - Behaviour Shield included.
...with 393 I can't get my computer to run with Behaviour Shield.
Windows XP Pro SP3