Hello Milosh,
This avgnt.exe error can be fixed either by re-installing the Avira security program and then reboot the computer.
You can also simply copy the same avgnt.exe file from another same operating system and replace the missing/broken file.
Perform the following troubleshooting methods:
Method 1:
I would suggest you to uninstall Avira security software then reinstall it and check if this fixes the issue.
Refer to the link “Uninstall or change a program in Windows 7” to know how to uninstall a program from Windows 7.
Method 2:
Perform System File Check to fix the corrupted system files and then check how it works.
Refer to the link "How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7" to perform System File Check.
Note: It may ask for Windows DVD to fix and to enable SFC to make more than minor repairs. Some files saved on your computer might get deleted when fixing corrupted files.