I think this started after I had a problem with Norton antivirus, so I uninstalled it and tried avast.
Almost every headache starts with Norton.
But, sure, each headache starts when you do not uninstall Norton properly...
Avast said that I had trojans and viruses and to delete the files it had suggested, so foolishly I did and after that nothing worked.
When a virus is found, the wise thing to do is send the file to Chest for further analysis.
You seem to have deleted a system (must have) file... :'(
Do any of you have any idea how to get everything back to normal?
The better will be a fully reinstall like RejZor and Eddy suggested.
If you don't want, try first reinstall Windows over your last installation, I mean, repair the Windows installation. You will need to 'update' through Windows Update page.