Hello. Just now I wanted to search a single board, this one:
Avast Cleanup (former GrimeFighter)
https://forum.avast.com/index.php?board=63.0
But it appears there is no way to search just this board. Is this correct? If so, please fix this. If not, please explain how to search a single board.
See screenshot… 
However to get that far you have to done a search and then select an Advanced search.
That said the forum softwares search function is pretty poor unless you are pretty good at compiling search strings.
It is probably better to use Googles search and add the Site parameter to serch the forum.
You might also want to look at this:
https://forum.avast.com/index.php?topic=164936.msg1240971#msg1240971