Video Hook Driver with UltraVNC stuffs things up for one reason or another. The crash for me was intermittent - sometimes I could get the logon screen up, other times whilst hardware was detecting. (1/2 gig,1.8ghz, gf mx440)
You don't have to uninstall it or stop using UltraVNC (don't expect to use SL across it of course - thats silly expectation) - but you need to clean boot your machine with the VNC service set to manual so that the Video Hook Driver isn't used (making sure you aren't running the service helper from startup or anything etc).
This solved my problem - however now I seem to have performance issues whilst running. (Uninstalling UltraVNC & associated hook driver doesn't help). Frequently being 'frozen' in place which I think may be connection related.