Don't change or modify anything until all personal data is safely
backed up.

As much as I dislike having to write this, recovery from an update is
one thing Microsoft (and some other vendors) do right with so-called
"recovery points". I had the proverbial nightmare earlier this year
upgrading a Vista system to SP1 -- it just wouldn't "take" and I later
discovered the problem was due to NVIDIA. Yet, after the SP1 failure,
the system was automatically restored to exactly the configuration it
was before the failed update. I was astonished the first time, and
tried it again immediately to see if the recovery would again repeat
from a failed update (which included a blue screen). It did. That was
simply amazing.