I hope I don't offend you by asking this.. are you sure that you did ONLY this (and in this sequence):
Change to 50, press OK button in same screen, restart IE? That should work (and does here )
What could maybe be a problem is your UAC settings. If this is what you did and it was not accepted... try the same with a elevated permissions as admin?
My Computer
At a glance
Windows 7 64b UltimateI7-2600 3.40GHz - testing various OC levels..2x 4Gb DDR3/1333GTX570 - testing OC levels