I'd attach the HDD to a USB caddy and using another PC, run a checkdisk /f and see if the disk is healthy or repairable.
If booting afterwards also fails, I'd stick the disk in the USB caddy again and copy the profiles and data off somewhere safe and then take a close look at the Default user profile and the All User profiles [1] as I suspect something is wrong here as it is at this stage where your PC is locking. You could always copy these two profiles off another vanilla machine [2] over the top of these two and try and boot again.
Failing that, use the USB caddy to drag off your data (if you've not already done it) and then rebuild from scratch. Windows 7 only takes 30 minutes to install from a DVD. use a new HDD too as their only £30 so not worth taking a risk with the old disk.