Something to do with the Intel SpeedStep technology that cause incorrect calculation...
http://support.microsoft.com/default.aspx?scid=kb;en-us;899525
Want to down the patch while not contacting PSS? Here the answer -> http://www.buit.org/2006/07/03/running-ms-virtual-pc-2004-smoothly-on-vista/
For those currrently working on MCMS 2001/2002 or the next version of WCM incorporate into Office 2007, this is the "must have" link under your favourite bookmark.
http://www.andrewconnell.com/blog/articles/MossWcmResources.aspx