system
30
Windows Defender can be uninstalled via Add/Remove Programs. So now the plot thickens cause your running Windows Defender along side Avast when your system is very very old. The more programs running in real time the slower your system. Especially with insufficient ram. The uninstall tool is to only uninstall Avast. Hence the Avast Uninstaller reference. The other uninstall tool he linked you to is to uninstall Norton and that is it.
Sometimes you may not be able to uninstall Windows Defender but you can disable its real time protection. Here is an article to follow.
http://www.howtogeek.com/howto/15788/how-to-uninstall-disable-and-remove-windows-defender.-also-how-turn-it-off/
Please again keep in mind that you can spend hours and days tracking this down. Possibly weeks. But sometimes its best to cut your losses and move on. Meaning looking into a new pc. You have everything to gain and nothing to lose. Just some friendly advice. When a customer comes to me with problems such as yours I never wanna take there money. Pay me $100+ to look at and repair there pc or for less then $400 buy a new pc. We will do our best to help you here. Thats what we our for.