There's something very wrong with their story.
First of all, you cannot see a virus or malware by using Treesize.
Treesize is a great program but all it does is scan your computer, to tell you what files and folders take up all the space.
Trusted Installer is a Windows function. You would see it at work, for instance, when your doing Windows Updates or installing some programs.
Nevertheless, we might as well check for malware while you're here, by downloading Malwarebytes and installing it (be careful not to accept the trial version while installing) then scan your computer.
If it finds something, it'll clean it up.
If it doesn't find anything, you can be fairly certain that those guys at your computer helpsite, are telling you lies.
All is well in Paradise.