Here's what you do:
1. When the NT AUTHORITY screen comes up, hit Start--Run, cmd, type "shutdown -a". The NT AUTHORITY screen will go away.
2. Reboot in safe mode (press F8 once the BIOS starts loading)
3. Apply the patch.
4. Run msconfig to check for any odd executables. avserve is one I remember seeing, as well as some randomly named executables. Remove them from the startup.
5. Reboot, put NAV on the machine, and do a full system scan. The latest definitions detect Sasser and its variants.