Application Host Service Has Stopped Working

I originally posted this on the Virus and Malware Forum suspecting that an AV product might be causing problems on two different computers. Thought I would appeal to a wider use base here to see if anyone has any idea what might be happening.

"On my older machine, which keeps chugging along with Vista with IE9 which can go no higher....

About every 30 to 40 minutes, it gives a "hiccup" and a message appears saying: "Application Host Service has stopped working." At the same time the icon for Panda Protection, my AV, briefly shows a red X which goes out after about 5-7 seconds. I press "Close" and thing's proceed normally until the next occurrance....these periodic interruptions can go on all day.

On the other computer, which runs Windows 7, I always have to manually start the Windows Firewall and Windows Defender, and Defender seems quite reluctant to do so at times.  Once on, they stay on. I vaguely remember this began when I started to use Panda as my AV.

Haven't received any Windows Updates on the older computer since a year ago last August, and don't think it, Windows Defender, is even active, although I still scan with it. Needless to say I don't use it for any browsing where security is a must, but scans with my other security programs including MBAM and Hitman Pro show I haven't picked up any malware on it. And Panda never finds anything either.

Just wondered if Panda might be causing these glitches. Am leery of using Avasta on this computer which has running for about eight years now, or if Avast will even run on it. Any thoughts, as always, appreciated"