Virtualbox high CPU usage - a solution

I've been playing around with Virtualbox recently to host an Asterisk PBX on my Windows 2003 server.  Asterisk (or more accurately, Trixbox) comes as an ISO and uses CentOS flavour of Linux.  However, I've noticed - as have lots of other people - that the Virtualbox process uses quite a lot of CPU even when nothing is actually happening.  After some searching and a few failed solutions, I've come across one that works very well. I've set up another virtual machine in Virtualbox, set the base memory very low (4MB) and set it running.  It doesn't boot up with any OS, it just complains and fails about not having a boot disc, but strangely it drops my CPU on the server from 17% down to a more normal 2%.  Trixbox runs just as smoothly as it did before!