That's because Virtual PC, the leading emulation program for running Windows on a Mac, doesn't support the G5. Microsoft, which acquired Virtual PC from Connectix in February, said a fix for the problem is not around the corner.
The new version of Virtual PC is expected within a year, about the same time as the launch of Office 11, the next version of Office for the Mac. More details can be found at CNet News.
And while we are at the Mac subject, Ars Technica posted today their Macintosh Browser Smackdown. OS X's bundled IE (old version) doesn't cut it anymore when you have other choices like Apple's own Safari, Mozilla & Opera.