The obvious cause to me seems to be the same problem that caused the antivirus error... and that is a viral infection.. So, what you need to do is scan the system with an alternative antivirus since it's obvious you already scanned with the installed antivirus. So, either install another antivirus or do an online virus scan.
you may also need to disable System Restore previous to the scan to delete the restore points folder being that viruses can hide in that folder and restore themselves after you have deleted them with the antivirus. If the virus scan doesn't help, do an antimalware scan with Malwarebytes Anti Malware.
Also delete the startup programs from the startup; Start\Run\type: msconfig and press Enter, go to the Startup tab and uncheck all the processes except the one for the installed antirivus,
next in Start\Run, type: %temp% and press enter and next delete all the user account temporal files and folders.
Also in the Run process, type "Temp" no quotes and press Enter... (this opens the C:\WINDOWS\Temp folder) and next delete the system temporal files and folders stored in the folder.
And to clean leftover system logs, registry junk, etc., install & run CCleaner.
If all of these steps successfully performed don't help, the problem may be caused by deffective hardware: Losely installed RAM module, bad Power Supply, bad Hard Drive, overheating processor...
A concrete answer to your question is; If your computer is running a weak and deffective power supply, (one of the symptoms can be slow running Windows) that can lead to motherboard damage, and also cause bad Hard Drive sectors that can lead to corrupt information... so make sure this is not the case.
You can easily test a Power Supply with a DC voltmeter... the correct procedure and voltages for every PSU to motherboard connector pins is available with a search for: "Power Supply voltage pinout" or "How to test a Power Supply" One example:
http://pinouts.ru/Power/atxpower_pinout.shtml