Hi z_o_o_m, I have the same issue. I was using version 5.1.3 until last night when I updated to 5.1.7.
It was working perfectly before the update, but now when I enter any value in the set max speed dialog nothing happens, if I open the dialog again is back at 0 (unlimited)
This happens on my Windows XP SP3 with avast and Comodo.
EDIT: I updated to 5.1.8 and it's working again