virt-manager "Cannot allocate memory" creating Win2K3 HVM

Configuration of dom0 is as follows
8GB memory
2 x dual core Xeon
kernel-xen.x86_64 2.6.20-2931.fc7
xen.x86_64 3.1.0-2.fc7
xen-libs.x86_64 3.1.0-2.fc7
libvirt.x86_64 0.3.2-1.fc7
virt-manager-0.4.0-2.fc7

I'm trying to install a win2K3 32bit HVM domU
using /cd.iso file local to dom0
LVM partition /dev/vgmirror/win2k3 as domU disk
bridged eth0 network
1024MB memory for domU (have tried with lower memory)

Fails with POST operation failed: (xend.err "Error creating domain:
(12, 'Cannot allocate memory')")

Rebooting the dom0 is to be avoided if possible due to other paravirt
domU guests running, any suggestions or extra info that might help?

I have managed to successfully create a win2K3 64bit HVM domU on an
identical dom0 server.