Bicycle Mechanics - What might cause a bike to pull to the right??

I'm helping to rehab a friend's old MTB before we release it back into the wild, and when I raod it, I found that if I took my hands off the bars, the bike pulls to the right. I tried it a couple of times with the same result.
Any ideas?? bent frame? bent rim/loose spokes maybe?

are the tires wide? maybe the front tire is not seated properly on the rim? could be that the headset needs adjustment.... or maybe something is bent, hopefully not though

Fork bent? If it's a suspension fork, one leg may be compressing more than the other or the slider is bent.

If the road you tried this on is crowned so you were riding on the slope going towards the ditch it would do this as well. Even a little slope will cause that. Try it again but this time ride back on the same side of the road (ie; against the traffic. Hopefully it's a quiet street :D) and see if it tries to dive towards the left. Even a strong crosswind can make a bike do this too, so take that into account as well.

If it's still aiming for the right despite switching directions then it may be the forks as suggested above or it could be that the rear wheel insn't centered in the frame so it makes the front track over to one side or it could be that the frame is bent from some previous crashing and that is forcing the tires to not ride in the same path and make them "tilted" compared to each other.

to recap:
bike has solid forks.
rear wheel seems to spin OK on the bike.
tried same experiment with a road bike and the road bike tracked straight for me, but the MTB is like a bad shopping cart.

Check the headset. I bet it is pitted. If it is, you will need a new headset.

Also, before you take the headset apart. Check to see if it is adjusted properly. Bikes with overtightened headsets will not handle properly. If the headset turned out to be pitted, it is most likely because the headset was over-torqued.

to recap:
bike has solid forks.
rear wheel seems to spin OK on the bike.
tried same experiment with a road bike and the road bike tracked straight for me, but the MTB is like a bad shopping cart.
A rigid fork could still be bent
The rear wheel can spin unobstructed but still not be centered in the frame
It's something in that particular bike alright.

http://www.sheldonbrown.com/brandt/pull-side.html

Also, before you take the headset apart. Check to see if it is adjusted properly. Bikes with overtightened headsets will not handle properly. If the headset turned out to be pitted, it is most likely because the headset was over-torqued.

http://www.sheldonbrown.com/brandt/indexed-steering.html

Rear wheel not seated properly.

You need to take the bike for alignment combined with balancing and rotation of the wheels.

Road is usually crowned , higher at the center line, for drainage..