I've been having this problem for a month or two. The first time it happened, I suspected Avast and was able to roll back the most recent Avast update - and bingo, my PC worked again.
Since then it's been happening occasionally. I can fix the problem by switching the PC off at the wall, which gets me the "Windows shut down unexpectedly" screen and I can "start Windows normally" from there. It's bloody annoying though.
I suggest people check to make sure they've installed Service Pack 2. I install updates regularly so assumed everything was up to date - but when I checked my history, I found that SP2 had failed to install. I've fixed that now so let's hope it works.
