hi
just to let people know. i tried to update my free avast today. seemed fine until very near the end of the install and then said
“launching executable file: kb970158_86.exe/quiet” and froze.
After forcing a computer shut down (task manager stop avast application did not work) and restart computer avast was actually installed except for the active web shield protection. I could turn it on inside avast but it just kept turning it back off again. thus i had the exclamation mark indicating avast was not happy about my protection. Other methods could not resolve the web shield not turning on.
i then tried uninstalling avast and cleaning the drive of avast references. install new from web page… but same thing again.
I decided to wait for this exe file in case it had a lot of work to do.
while waiting i decided to close firefox.
then later still waiting i restarted firefox. immediately a message said that another software program wanted to update this application. i said yes to continue, thinking it must be avast updating firefox. As soon as it did this avast moved on and completed the install 
i am using windows xp sp3
hope this helps
(edit: kb970158 for other possible search references)