Funny that you mention that, I had run into the same issue a few days ago. My guess is that it's an installer/compatibility issue.
I'm hoping a plug-in that works with VirtualBox 4 is coming soon and would be interested in even test driving a beta (hint hint
). Reading this post
has my hopes up for the time being.
I use VirtualBox to run a 64bit Linux sandbox/dev environment and wanted to run XP mode to test different versions of IE and other apps on XP and ran into issues with Windows Virtual PC not being able to run since VirtualBox was already running a vm with h/w acceleration (VT-x/AMD-v). Nothing like watching 2 virtual host apps fight for resources.
Scouring the web I found VMLite XP Mode and it looked like a good solution to use the two together. I like VirtualBox because it feels more "light weight" in comparison to VMware Server. I would have went with Windows Virtual PC, but it had no support for 64bit systems, which I find to be odd.