Im just gonna put this here and let you decide:
http://www.youtube.com/watch?v=Th71nkLc0Z4
That said, you can accurately fire a good bow at about 45 rounds a minute even while moving...thats allot of dead zombies fast. Lets say you only get half that since you want headshots...your still taking out zombies as fast or faster than a blunt weapon while using less energy and staying much farther away.
Ive seen a pistol grip type repeating crossbow with a bayonet mount (slot between grip and bow with a two screw clamping device), sticking out the front kinda like this:
using a compund mechanism and pull handle with about 20 bolts gravity fed weighed about 5-6lbs. Its was a simpler compound bow kinda like this:
maybe 50 lbs draw not entirely sure the firing weight (is it double?) The hand grip was almost on the front which gave you much better control with the bayonet (like a 24" double edged knife, easy to slash/stab with) and a longer pull on the string.
Theres quite a few variations that will work. Ive seen more ordinary bows with front guards/blades as well for clubbing/slashing (essentially a bow built onto the grip of a sword/mace)