Can't Log On..Blue Screen Error..Help :(

Hi everyone, first, I'm new so my apologies if this thread is in the wrong category. So I rebooted my dell laptop and got to the black screen where there were ''start windows normally'' and ''launch startup repair'', I chose start windows normally but it just rebooted and same thing happened, so I chose to launch system repair when it finished running same thing happened,it rebooted again and same, so I started it in safe mode and same thing, so I tried chosing from advanced boot options ''Last Known Good Configuration'', and the blue screen appeared which reads ''STOP : c000021a fatal system error'', then guess what happened, it rebooted and same thing happened .
So as you can see I cannot in anyway log on.
Also, I neither installed new drivers nor updated any.
Please help!
A million thanks to any help.
:) :) :)

Edit; the reason I first rebooted my laptop was because avira free antivirus asked for a password (which I didn't set) for disabling the program and for accessing the admistrative section..so I googled and found this:
''Start the computer in safe mode then go to
C:\ProgramData\Avira\AntiVir Desktop\CONFIG
• Click with the right mouse button on the AVWIN.INI file and open it with the Notepad
• Edit the file under the item [COMMON] as follows:
Password=
SecurityExtension=0x00000000
• Save the file
• Restart your PC
• The password protection for Avira has now been removed''
And that's when it happened

please follow the instructions at the top of the page in the bsod forum. here is the link to that:

Blue Screen of Death (BSOD) Posting Instructions

If you have a disc drive then try booting into a bootable Windows 7 CD if you have one.If it still comes up with the blue screen of death then it has something to do with either your motherboard,or memory or hard drive.