Hi!
Using VLC 1.1.11 on Windows 7 SP1 64 bit, I noticed it uses much more CPU than Windows Media Player when playing MJPEG movies (in AVI container).
VLC uses about 66%, while WMP 20% for the same file.
This is similar on two PCs.
One netbook with Atom N270 (1.6 GHz one core, two threads) and one HP Microserver (AMD N36L dual core at 1.30 GHz), both having Windows 7 (netbook 32 bit, server 64 bit).
On another older PC, it is the opposite. The VLC is using 2x less CPU than WMP (WMP 11 on Windows XP in this case. On Win7 it is WMP 12).
This all with the same test file: 640x480 30 FPS AVI file, made by a Sony DSC-W530 photo camera .