I've been a long time fan of VirtualPC.
On the Mac it's let me work on projects than needed Windows (have you ever tried to get SQL Server running on OSX? It's the only way!)
On Windows it's let me goof around and try out various configurations and scenarios (and open really dangerous attachments) without worrying that I'd be saying goodbye to my main machine (For instance I ran Office2007 Beta in a VirtualPC for a couple of weeks before upgrading it to my main environment)
So, what could make VirtualPC better? Well... if it could miraculously take up less space I'd be very happy. But failing that it's always performance (especially on the PowerBook which was limited in raw cycles and RAM). And for Mac users support for the new Intel based hardware would be great (althouth the folks at Parallels seem to have got that market sewn up now!)
On the PC side of things... well, there's one thing better than a good value product... it's a free product. And VirtualPC 2004 is now a free product. And unlike so many "special offers" on the web it's actually the latest version that's free. And there's a promise that the next release will also be free.
So there's no excuse not to have a safe, secure, isolated test environment for trying out those things that might just bust your machine!