Before I read Jeff's post, I was thinking timing as well. Check that, make sure the timing marks, and the timing itself, is where it should be.
Also, like Jeff said, if the pressure plate wasn't tightened correctly, it'll do that to ya. Basically what happens is you have the clutch disc "out of round" (centerline NOT perpendicular to the crankshaft) and it's grabbing ahold of the pressure plate.
It's something simple, don't get freaked out. You'll find it!!!!
