virtualbox.org

I think I've had this happen on my XP host as well, but I don't use it anymore.

If I start a VM from the GUI, and switch to another application (Firefox, for example)... Virtualbox always seems to crash. VB seems to crash if it's not the active window.. I'm assuming right before the XP logo screen comes up. Not a major issue, just sort of an annoyance.

Other than the PC being out of resources at the time I can't point you in any direction. I have not heard anyone that has had that problem that I can remember either.
The only other thing that I could add is something that I am sure you already realize. Windows 7RC as host is not supported yet, which could be what is causing this glitch. Give it a bit more time and see if it improves any as they add updates.