Try booting up in safe mode. When starting hit F8.
If you can get to your OS restore your system back to a time you know it worked.
Use a restore disk ad restore that way if you can not get to the OS.
You will need to set your CD drive as the first boot devise.
When you get everything working get rid of AVG.
It caused boot problems for me.
Get MC security essentials. 100% free and it works very well.
http://windows.microsoft.com/en-us/windows/security-essentials-download
Happy Computing!