sounds just like what happened to me. Here is my recent support ticket:
Hello,
I've been using Avast free for a long time with no problem. Yesterday, I installed your latest program update and my system (HP Touchsmart, Vista Home Premium 64)crashed -- Blue screens of death, automatic CHKDSK, failure of other software (Diskeeper Pro, Google Chrome, etc), and loss of icons in the System Tray). When I uninstalled Avast, system performance improved.
Have you received other reports of problems with the new version of your program? I really like it alot and would like to return to it as soon as possible. Do you have any suggestions regarding how to install safely?
Thanks...
You are not alone