OK, I think I solved this one simply by running aspnet_regsql.exe -S ServerName -E -A all -d DBName and, I think, reinstalling the needed stuff. But this begs a question from me: shouldn't these things have been installed when I first installed YAF?
And, now I have other issues to resolve, some of which still beg the question: would I be better off using the standard asp member providers over YAFs? I believe I read here that YAF's providers are there to provide for ease of upgrading, more than anything else.
Now on to my next issue, which I will be posting about shortly.