I have had XP guests sometimes hang when doing a Windows-Update-invoked reboot. Both XP and 7 hosts, different versions of Virtualbox.
The guests get to just before the VM window would shrink to 640x480. They stop responding, no disk or network activity, just XP's peacefully-running light blue (not BSOD blue) background. If I pause/unpause or save-state/restart the VM window turns black and gets a grid of greenish yellow lines filling half the window. The VM does nothing else and I have to reset it. Then it boots up normally.
Two XP guests sometimes exhibit this behavior for now. I used to have this problem in a third guest, but it hasn't shown itself for a long while, despite still getting the same Windows updates as the two present offenders.
I have searched and found references to audio being the culprit (one guest has audio, one used to but does not now). Also, "absolute pointer" has been fingered, but I never use that setting and neither guest has ever had it turned on. I hunted thru the bug tracker but couldn't find a pertinent post, only one post actually mentioned Windows updates and the poster didn't put in logs, so the post had no activity.
So, here's the logs:
XP guest: "XPnet"; XP host, audio on, no absolute pointer, VB version 3.1.2 (this one runs all the time with no scheduled reboots, I think all the logs in the zip are from Windows-Update-induced hanged reboots)
XP guest: "Email Tester" Windows 7 host, audio used to be on but now disabled in VM settings, no absolute pointer, VB version 4.0.16 (this one is shut down by a host-induced Vboxmanage command every night; only the most recent log was a Windows-update-induced hanged reboot, the earlier logs should be normal shutdowns. I've also had this VM running on version 3.2.12 on a different Windows 7 host, with the same behavior)
The third guest that used to have this problem also was XP, audio on, no absolute pointer. It just had a Windows Update same time as the other two, but did not hang during reboot. Here's this VM's last Windows-Update-successful-reboot log for reference:
Guest Additions are current in each VM. I have not seen if other causes for in-guest-invoked reboots cause hanging, and I haven't had the hang happen when I tell the guest to reboot from the guest's Start menu or a "Shutdown -r". It seems that Windows Update is the only rebooter that causes this to happen that I've noticed so far. And I've not experienced Windows Update having a problem rebooting the physical XP PCs I have at home.
running version 3.1.2, 3.2.12, 4.0.16, 4.2.16