About dual cores

Hi,

I have an Athlon 4800+ X2, running Win XP SP2. I do some video encoding
using SUPER, which is a GUI to Mencoder. Process Explorer from Sysinternals
show that Mencoder has only one thread, however the system view shows both
cores loaded at about 50%.
How come that a single thread application can run on both cores?

Thanks for any explanations.