I got a crash immediately on trying to start XP guest after upgrade too (64 bit 8.04 Ubuntu host, upgraded from VirtualBox 2.2 - guest extensions previously just deinstalled from XP guest). Tried restart and a few other things that didn't help.
It *did* boot after I disabled CDROM, sound, LAN, and set VT-X (don't know what was the magic ingredient).
After that I re-enabled CDROM, sound, LAN and all working fine. Also tried multi CPU support on Quad core Q6600, but XP was incredibly slow so had to change back to single core.