Hi all - How do you get both processors of a two-CPU machine rendering?

(Version 4.5 on Windows 2000 Pro - More an O/S question than a Maya question!)

I have set the 'use both processors' option in the render globals box, but still, when I open the Windows Task Manager while rendering, I only ever see the CPU usage at 51%

Am I wrong to expect to see both CPU Usage History boxes flat-lining at the top, thereby satisfying me that both CPUs are rendering?

Dan