How interesting. Even with all the advanced recovery options in Win 8.x, you still have to manually
rename registry hives to get the advanced repair options to work.
A couple of weeks ago my Vista PC refused to boot, somehow avast had become corrupt.
No normal boot, no safe mode; last known configuration didn't help either.
Essexboy in the virus and worms forum gave me a list of instructions to be used with FRST.
This list did a partial uninstall of avast, so that when Windows starts, it no longer attempts to load avast.
Once this was done, Windows booted normally and I was able clean it up.