Dave I've had this happen three times now with XP Professional, and ended up
trashing both Vista and XP installations all three times and reinstalling
them both from scratch. I have done all the replacing hal.dll and
ntoskrnl.dll suggestions, repaired the XP installation from the CD (it
succeeded but made no difference), and now I'm up to "recovering a corrupted
registry that prevents Windows XP from starting" (see
http://support.microsoft.com/default.aspx?scid=kb;en-us;307545 ). Maybe
they just don't get along ... I'm starting to think that the problem is
either with Vista's disk indexing or with the antivirus program i'm using
(AVG free edition) corrupting my XP installation as it's closing down. It
always seems to happen when i have just logged off the Vista installation and
am trying to go back to the XP installation - or maybe the XP installation
was in the middle of some kind of update and I screwed it up by switching to
the Vista on reboot instead of going straight back into XP. I don't know if
any of this will help you, but you are definitely not the only one it's
happening to
