What are your exact settings of the resident protection, specifically the Standard Shield provider?
Do I understand correctly that your PC locks up when you change skin of the avast! Simple User Interface?
The default settings, with the exception I changed the skin. Also, one day, unfortunately, Avast caused Windows XP to lock up even when I didn't change anything, including the skin. One day, Windows XP locked up on me just after installing Avast, even when I reformatted the HDD, reinstalled Windows XP and didn't change any skin. I doubt my hardware is faulty, because the above symptom never occurs when Avast isn't installed. I made sure to reboot Windows XP after installing Avast.
BTW, the Windows XP kernel itself may have not crashed, when I get the above symptom, but I loose control of my PC, because even Task Manager will fail to respond and thus I'm required to push the reset button on my PC.