What a bizarre question - why would you want to? the simple answer is that it depends on how hard you can hit yourself (and the position - i may be wrong but usually being knocked unconscious is through a bump to the back of the head)
chinese whispers.. mis-information.. jenstar didn't say he 'wanted to' darth vader - just 'Is it possible'.. so, what a bizarre answer is more correct.
I would think it is very difficult - your own in-built defences would prevent you from hitting yourself that hard. In the same way you cannot fall forwards without bringing up your hands to break your fall, such defence mechinisms are mentally 'built in' and as such very difficult, although not impossible.