Startup Problem

As I mentioned awhile back I recently switched from Norton AV to AVG. After
a few twists and turns and advice from this NG, things are running smoothly.
But there is this one problem...

When I add a program to startup, the machine freezes, both mouse and
keyboard. I found this out by defragging, because to do this I have to
disable GoBack and reboot, defrag, and then reenable and reboot. I tested
the various stages of this process by rebooting, and it's definitely the add
back, not the disable or the defrag, that's causing the problem.

I can get back to normal by hitting the reboot button and going into Safe
Mode, and then rebooting again from there, so the condition isn't crippling
but you'd think it ought to be fixable.