How do I maximize security for XP even after support ends?
In MBAM and SuperAntiSpyware you have made two excellent choices. While OnlineArmor Firewall (OAF) is powerful it clashes with many other programs. Years ago I found that out the hard way - I was using Avast Free Anti-virus and I added OnlineArmor Firewall and that crashed my system.
I am going to quote the obviously biased manufacturers of AVG Free.
"AVG AntiVirus Free not only gives you the top-quality antivirus protection and security features that you'd usually expect only from a paid product, but added privacy features for 2014 help keep your personal information safe both online and on your PC, and the new integrated File Shredder permanently deletes sensitive files to keep them from falling into the wrong hands. Add to this our Anti-Spyware Technology and WiFi Guard, and you can be sure that wherever you are, at home or on the move, we're taking your privacy seriously."
It would be an excellent addition to your system if OAF is amenable to it. If I were you, and assuming that you backup to an external hard drive, I would download but not install AVG Free. Then I would do a complete backup after which I would install AVG. If you aren't backing up your system you should be - otherwise if your old hard drive fails you will lose everything.
However, by now you're probably competent enough to rescue the system in the unlikely event that they cannot co-exist. AVG Free is a superb program and not as fussy as Avast.
*****This last part is extremely important. Never download any file from CNET. If you do, when you install, all sorts of garbage enters your system. A new toolbar is added and you will find that it has changed your homepage as well as your default search engine. Try to download from the home page of the developers who have written the program. During the installation there is an excellent chance that the ever increasing propensity of "free offers" will be offered to you. Sometimes you have to uncheck a checkbox, sometimes you have to check Decline, etc. Unlike CNET which forces these on you, you will have a choice. Don't accept anything other than the main program.