try starting Windows in safe mode. If it starts in safe mode, uninstall MSE, then try starting Windows normally.
There are two versions of MSE:
a) a version for 32-bit Windows
b) a version for 64-bit Windows
Download the correct one for your Windows version: http://windows.microsoft.com/en-GB/windows/security-essentials-download
Or don't install MSE at all - - use an alternative antivirus.