Install breaks Windows Vistat Startup

AN ODD PROBLEM: I am installing Comodo 64bit on Windows Vista 64bit SP1, AV: Avast 4.8 Pro, Windows FW, 8GB, ATI X1850 PCI-x card, 2-SATA300 drives, Tyan 2877 motherboard
After reboot and signon, Vista stops in the middle of its startup and I am left in a state where the screen is black, with a pointer. I can bring up task manager, but nothing is running and CPU flat lines. except for a few windows tasks nothing else is running. It seems to be in a stoppped state, with the exception of task manager.