Disappointed with 6.0.1125

I had been using 5.1.874 without much issues refusing to succumb for an update.
Now the curiosity got better and installed 6.0.1125. First I tried over the top install.
Next I found very innocuous program like FSCapture would not run , Netlimiter is killed silently in the next start. Worst even FIREFOX 5.0 setup would not run.

The next obvious step was to disable AVAST and all the programs ran without a hitch.

Next I unistalled AVAST and did a very fresh install of 6.0.1125.ALl the problems mentioned still persist.

Should I go back to 5.1.874 or has some settings gone wrong??
I am using WIN 7 home edition SP1.
Thanks in advance for any help.

What you mean by clean uninstall, did you use the avast uninstaller tool?

Download Here:
http://files.avast.com/files/eng/aswclear.exe