Backdoor Trojan

On my wife's Win7 PC she runs Symantec Endpoint Protection and today a SEP notification msg started popping up [SID: 29106] System Infected; Trojan Backdoor Activity 152 detected.

She also noticed that Google Chrome wouldn't open/run so I did a full AV scan using Malwarebytes (also their rootkit tool), Kaspersky's Free Scanning Tool (and their rootkit tool), Rogue Killer and none of the utilities found any issues. I uninstalled Google and reinstalled it and now it is working.

Everything seems to run OK (PC boots a bit slowly), however every 20 or 30 seconds the SEP "[SID: 29106] System Infected; Trojan Backdoor Activity 152 detected" notification pops up.

Have also tried - disconnecting (by temporarily disabling the LAN network adapter) from the internet and I no longer get the SEP pop-up notification - but it returns as soon as i reconnect to the Internet.

Would appreciate any advice on exactly what this msg is trying to tell me and how do I fix what ever the problem is.

Very frustrated - something seems to "living" in my wife's PC and I'm worried about how safe it is for her to use the PC.

Many thanks - and stay well.

 

Getting assistance here
https://forums.tomshardware.com/threads/backdoor-trojan.3617006/