First of all, go to advanced boot options, and see whether last known good configuration works or not
can you boot into safe mode? If so, try to do system restore, if that doesn't work as well, try to do a clean boot, followed by an advanced clean boot and a hardware clean boot.. these steps might help ..
clean boot : http://support.microsoft.com/kb/310353
advanced clean boot : http://support.microsoft.com/kb/316434
hardware clean boot :
Follow the steps below to perform hardware clean boot
1. Right click on My Computer -> Properties
2. Hardware Tab -> Device Manager
3. Right Click on Sound Card, Video Adapter, Network Adapter one by one and click disable
4. Reboot the computer. If the problem is fixed then you can enable the hardware devices one by one to find the exact device that cause the problem
If these steps don't solve the problem, unplug any external devices from your computer and see whether that resolves the problem .
If nothing works, try doing system restore from winre, if that doesn't work as well, you'd have to do that manually,
try these steps,
go to WinRE) open folder C:\Windows\System32\config
and rename these files:
(don't change COMPONENTS file because system don't store any copy of this file,BCD-Template system will reconstruct automatically when you boot into WinRE)
Next navigate to C:\Windows\System32\config\RegBack
folder and copy all items to C:\Windows\System32\config
Next boot into WinRE and try again restore system to previous checkpoint.
The last option would be to format your root drive and install the OS again..
Hope that helps, and good luck