was looking at some of Dave Canterbury's video's, saw one of his instructors doing a video on a simple crossbow. with some tweaking you could probably make an excellent quality one, using just the trigger design. if you look at around 15:10 you see a picture of the trigger system.

crappy explanation (read masiaka's farther down the page):

if it were up to me in a non survival situation i would cut off the bottom half of the log except at the trigger, to make it much more light weight. i would also add a small handle behind it, and maybe even a stock. the arrow holding mechanism would probably be replaced for a nicely carved/sawed out small board, that is attached on top and held down with a hinge and maybe a spring (or just a piece of wood wedged on the other end of the fulcrum)

http://www.youtube.com/watch?v=bVCqdKlWetk&feature=channel_video_title

Think of it like this:

• There is a notch carve out of the stock for the string to rest in when you pull it back.
• In the notch there is a hole drilled right under where the bowstring is.
• In the hole is a pin that is attached at the bottom to the middle of a paddle and the front of the paddle is hinged to the stock.
• This paddle is the trigger.
• When you push up on the paddle it pushes the pin up.
• When the pin slides up it pushes the bowstring out of the notch.
• Once the bowstring slides up out of the notch it hits the nock of the bolt and pushes it forward.

This is the trigger used on the crossbow my father and a friend made for me when I was in my early teens, over 3o years ago
Both the old boys had reached their 60s back then, and had picked up a trick or two along the way.  
The stock they simply made from a solid board with tongue and notch(?) so the bolts fit in the notch.  I remember it as working fine.
timann