Here's an answer previously posted for the same question:
Two anti virus can conflict each other and make your computer slow. It's never advised to run two anti virus on the same machine.
Hi Chris, welcome to Trend Micro Community!
As a matter of fact, Windows Defender will be disabled automatically when installing Trend Micro Security software. This default change is to avoid conflict since Windows Defender is a Microsoft's built-in anti-virus protection and having more than one anti-virus program usually causes a conflict. So, as @woodartisan55 and @Jhja mentioned, it's not advised to run two antivirus programs on the same machine.
For your firewall question, Trend Micro Security does not have a firewall feature. But you can refer to the Microsoft support page below to enable your Microsoft Firewall.
Turn Windows Firewall on or off
For the "Potentially harmful software detected" message you've got, as long as the Trend Micro program is running, then you should be protected from Malware. We assume that this message might be due to background internet traffic from programs that you've installed unknowingly.
These programs, commonly called potentially unwanted applications (PUA), are typically bundled with other freeware, and installed as an option most people do not uncheck during installation. Please try the following online solution doc to check and remove any PUAs on your computer first:
Uninstalling Potentially Unwanted Application (PUA)
Some common PUAs are: SlimCleaner Plus, iLivid, Inbox Toolbar, MySearch, Search-Results Toolbar, TuneUp Utilities, and many others. If you found at least one PUA on your PC, it is likely that you have other PUAs installed.
Please let me know if you have more concerns or questions by replying here. Thank you.
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