if you installed this McAfee Security Scan Plus as a type of 'Add-on' to some other software you installed, for example I know that Adobe Flash updates often offer additional tools that you can either accept or op out of.
In fact that example is confirmed here; http://techie-buzz.com/softwares/mcafee-security-scan-plus.html
If so, then while these add-on options can be useful, often they are thought of as intrusions and, if I may say, sneaky, We don't always see the offer during the installation procedure, or in the case of Adobe, we don't often see the small option box on the web page, and more often than not, these options are already ticked and so they are not opt-in options but opt-out options. We have to opt-out if we don't want or require the extras, and that is what we miss.
What I am asking really is this. Did you specifically want this utility to help you manage the security of your system, or did you suddenly find it installed and so decided to use it? It's an important question because if you specifically wanted it, then fine. But if it was installed without your knowledge and you were, previously, satisfied with your computer's security regime, has this utility placed doubts in your mind?
And that leads to the same question in a different way. Are you satisfied with your existing security tools at present? If so why do you need this added tool?
You say that this McAfee utility doesn't seem to recognise your Avast anti-virus scanner. I assume you are using Windows, and if so, does the Control Panel > Security Center show Avast as installed? If so then I would personally trust Windows, and not this tool.
Mark