I had seen this with beta 5 on a VM that was originally created with beta 3 and then successfully updated to beta 4 and then beta 5.
For me, I could almost always BSOD the XP VM if I tried to use the Windows Explorer app -- the one you get by double clicking the Computer icon in XP.
I got so annoyed that I deleted the old VM and created a brand new XP install using beta 5 from my XP/SP2 install CD.
I was even able to authenticate the new VM with Microsoft with no hassles.
Since then things are relatively stable but I am still having problems with the Explorer app sometimes leading to a hung VM, especially if I use it after I suspend and then resume the VM.
For now, I am swearing off suspends and resumes in favor of a Windows shutdown and then needing to re-boot. Seems slower but safer.