A recent Windows 7 installation started crashing with a Bluescreen of Death after installing the qlx driver inside the guest. Switching back to the vga model allowed me to boot it again:

<video>
  <model type='vga' vram='16384' heads='1' primary='yes'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>