Solved Anti-virus software compatibility question

My son's laptop, running Vista Home, tends to run without much protection, and today I was troubleshooting it and decided to do a Malwarebytes Anti-Malware quick scan, then check in the "Security Center" in the Control Panel to see what else was doing any active protection. It seems that "Windows Defender" is present, and was enabled. The Firewall option was not enabled, and a message said that there was no anti-virus program being used. (Isn't Windows Defender an "antivirus program"?) So, I decided to install MSE as an anti-virus program. The installation said to remove any other antivirus programs that are active on the computer. I did not do anything, since I looked at the list of programs, and it seems that the only program vaguely like that would have been either Windows Defender or MBAM. I've been told that MBAM does not interfere with antivirus programs, so I left it. I did nothing to the Windows Defender application. So, I downloaded and installed MSE, and it is now running its "1st scan". During the process, I noticed that Windows Defender was still there, so I clicked on the icon in the systray, and closed it, choosing "exit". I wonder, should I uninstall Windows Defender, now that I have MSE installed? Will they interfere with each other? Also, it told me that the firewall was not active, so I made it active. My son likes to avoid any programs that slow down the computer, so he avoids anti-virus programs. His system, again, is Vista Home.

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