Is it the google calendar sync which is the problem, rather than the display of the calendar in the igoogle page ?

I’m a bit confused as having uninstalled avast completely the problem still exists, avast doesn’t change any settings as such but would monitor, scan all HTTP activity. Plus the fact that it works on the other system is a bit confusing.

Did you also run aswclear.exe for avast 4.8 in safe mode ?
If not I would suggest doing that before a clean install of avast6.