How do I restore "sleep" mode on pc, after I connected media Center on Xbox?

A while back I connected Windows Media Center on my pc with my Xbox. I remember there was an option to always keep the pc “active” so that media Center would always be available on my Xbox if it would turn in sleep mode.

Now, whenever I put my pc in “sleep” it never really goes into that mode, which is annoying.

How can I restore all this?
My OS is Windows Vista.

I hope I explained it good enough for you to understand =]