Booted fine the day before, turned it on today and it got as far as the black screen with the moving Windows logo and just sat there. For hours.
It’d boot into safe mode with no problems.
Eventually I found this forum post which suggested to try booting into safe mode and then uninstalling either the graphics driver or the antivirus software. I tried the graphics driver and it made no difference.
Then I uninstalled the antivirus, which was AVG 2012, and that resolved the problem. PC is booting fine now. So I’ve installed the free Microsoft Security Essentials instead, which I use on all my other PCs anyway. Phew.
Edit: Just found this interesting article where AVG themselves tell you how to disable and then uninstall their product if it stops your PC from booting! Nice. A quick Google search shows that AVG causing this problem is not exactly uncommon…