Hi David,
I would guess that your rig has been moved. Before you zero out the joint’s weighting, try selecting the the skin tag on your mesh and clicking the ‘Reset Bind Pose’ button (or the ‘Reset Pose’ button in the Character Object’s Animate tab, these are the same function.) This will reset the rigs position to it’s starting position. Now you should be able to zero the joints.
Remember that even after this fix, if you had setup keyframes your joints may pop out of place again. In that case, you can either find a place in the timeline where the joints are in their ‘reset’ position, or record a keyframe to the character hierarchy immediately after clicking ‘Reset Bind Pose’ again.
Let me know if this helps. If not, please upload your file (ZIP’d, or RAR’d.)
Andy