On XP you will have to look at an alternative to MSE..... Not really sure what to suggest on that other than if it is a business use machine and needs XP then use it just for its main need and not as a casual surfing machine. Best to just keep XP off the web now since there are no more operating system security flaw updates or anything for it anymore. It is just going to get more and more vulnerable to security breaches over time. That pcmatic program does claim to keep XP secure though. I hate some of their commercials and have not tried it but it might be okay software. Be careful though, software that claims to do too many different things, just like trades, often means master of none.... If it has tools for cleaning up your registry, deleting temp and uneeded files, etc there is a chance it might not recognize your line of business software and it might do something to break your software. I'm assuming pcmatic does do registry cleaning and whatnot since their ads claim to increase your computers speed.
Already mentions of the very same solutions we use, MSE and Malwarebytes. Dedicated programs with a purpose. For general system cleanup I also recommend Priform CCleaner which has always worked great for us. It excels are cleaning garbage out of your systems registry and finding unneeded temp files and whatnot while keeping important temp files. However as of late their newest versions are getting more intrusive with trying to put on active monitoring. You have to be careful of that sort of thing with most any software though. ALWAYS pay attention during installation of programs and check the "custom" installation options making sure to unselect optional toolbars, browsers, unneeded features, etc. You might want active monitoring though, and on some other machines that others use I would as well. On my own, I want as little running all the time on its own as possible to keep the system resources as free as possible. If I want to update it, I'll check for updates myself, I do not want yet another program running 24x7 just so it can check for updates for me.