On demand is not what modern AV is for, really. ClamWin still does this, I think?
This came up not too long ago, if you go back over recent forum history (last month or so?) someone from Avast! team mentioned you could run command line Avast! (ashquick.exe) from some kind of barebones install, I forget why the guy wanted to do it. But it was not really recommended. Avast! is designed for pro-active defense.