[CentOS] guest vms crash host systems

I built guest vm's one for Windows 7 and one for Windows XP using the
virtual machine manager on a  just updated to centos 5.7, and they are both
crashing the host machine. They run only  for a few minutes, but suddenly
freeze, crashing the host.  There is no networking. No X. No way to drop out
of X. The only way out is a hard reboot. I don't see anything in the logs --
messages or libvirt logs -- immediately before the crash.

I haven't found anything like this on the web or on this list. The
workstation has two xeon E5410s. I noticed that both the kvm-amd and
kvm-intel modules are loaded, but don't know if that would cause a problem.
I had an ati firepro graphics card in the machine, but suspected that might
be the source of some conflict, and I put in an Nvidia card.

The vm's were built with all the defaults. The configuration is just about
identical to vms I have running on a smaller machine with a dual core
Athalon.

Thanks for any suggestions.