Online Armor for the most parts fine but it has caused some users grief after avast program updates and that is something you have to watch out for.
This no longer holds true with Avast 5.0 and the newer versions of Online Armor (OA). I have beta tested OA with Avast on XP and Vista in all versions for the past almost 2 years. OA also leaves no remnants behind on uninstall and gives no constant pop-ups while giving you the option of using it as a novice or advanced user.
PCTools also runs smoothly with Avast, however in the advanced mode does give you constant pop-ups (search in this forum). Uninstall can leave many remnants.
Since everyone's machine and set up is different, I would recommend that you trial a software to see how you like it and more importantly how it plays with your other software on your system. Just remember before you install, know how to uninstall and make a snapshot/back up/system restore of your system as a "just in case."