Missing DLL Issue :
Download the DLL file here :
vssapi.dll download - free dll files
To be safe, drop it in the c:\windows\system\ and c:\windows\system32\ directories. Reboot if necessary. This SHOULD resolve that issue. Your filepath and/or drive letter may change based on your system configuration.
Display Lockups :
Have you downloaded the latest display driver for your video hardware yet? What video hardware are you running? Driver updates are the first place to start with those type of issues. Ensure you uninstall the old one first via it's uninstall program if one exists for it (nVidia and ATI products will have an uninstall option)
Windows Update :
Can you either post a screenshot of the Error or a more descriptive error message if your presented with one? Word for word is best if possible
Windows Defender :
I have no idea how to resolve the issue. Try temporarily disabling it and see if that resolves your problems. I disable mine but I'm a power user and don't have much use for it.
Run MSCONFIG from either the "Run" option in the start menu or use the search box in the Start Menu. Once in, click on the "Services" tab. Search for "Windows Defender" remove the checkmark next to it and click "APPLY". Next go into the "Startup" Tab and remove the checkmark from Windows Defender. Click OK, Reboot. See if your issue persists.
You can always re-enable it later by following the same steps in reverse, adding the checkmark back.