Whew! You are about to expose how ignorant I am here. I will try my best to answer your questions.
Yes, I restored my computer to a previous state when it failed.
Yes, I did update the Windows Defender and run it.
I've never heard of a registry cleaner and am hoping since the computer is working ok now I won't have to do it!
I also uninstalled the Avast that wasn't working, reinstalled one I had saved, 5.0.418.0, updated its definitions and then updated again to the 5.0.418 program, then scanned my computer. There were 2-3 instances where Windows update suggested updates that required a reboot after installation. It was on the reboot that I got the 'service stopped' message. Each time I uninstalled, then reinstalled and reregistered Avast, updated the definitions and scanned my computer. It was when I started searching the internet to find out what was amiss, not thinking it was the Avast, and found this forum, that I realized it might be Avast and not Windows update or my computer that wasn't working right.
1. Yes, I started out last fall with Avast 4.8.1351.0 Home Edition. I then updated to 5.0.418.0 when it was offered. Then came 5.0.545, which is the one that I had so much trouble with. I know for certain the 4.8 one gave no difficulties. I 'think' the 5.0.418 was ok. I also 'think' I began to experience problems right away with the 5.0.545. By the way, I have two other computers running XP which have the 5.0.545 and I've had no problems, but I don't use the Windows update on them any longer as they're really, really old!
2. The only firewall I'm aware of is the Windows firewall which is always on. Info about my system. Hmmmm ... I'm not sure what you need, but here goes. Info listed under system information:
Manufacturer: Compaq-Presario
Model: NY461AA-ABA CQ5107c
Type: X86-based PC
Processpr: AMD Athlon 7550 Dual-Core Processor, 2500 Mhz, 2 Cores, 2 Logical Processors
Bios: Phoenix Technologies, LTD 5.49, 8/6/2009
SMBIOS Version: 2.5
RAM: 3 GB
Total Physical Memory: 2.87 GB
Available " ": 2.03 GB
Total Virtual Memory: 5.97
Available " ": 4.53 GB
3. No previous AV software.
4. No, I didn't know I should reboot twice after uninstalling!
5. Yes I have done a full scan. I don't know what a boot scan is, so no. I have started using Spybot Search and Destroy in addition to Avast and Windows Defender recently. I don't know what MBAM is. I had started to download AVG and Microsoft Security Essentials but haven't installed them yet and am hoping by using the version of Avast that's been working, maybe I won't need to.
Once I stopped trying to install Windows updates, the problem hasn't recurred, which has been about a week. Everything seems to be working as it did before. Before reading your note, I did uninstall Avast 5.0.545 and reinstalled a saved version of 4.8.1351.0, which I am perfectly happy with. I updated the definitions and found the definition version to be the same on this computer using 4.8.1351.0 as it is on the XP which has the 5.0.545 version which I updated today. If there's no difference in the actual definitions, it seems that the older version of Avast will suit my needs just fine since I never had any problems while using it.
Overall, unless you recommend otherwise, I think I've fixed the problem I was having to my satisfaction. I totally appreciate the time you've taken to address my problem and hope that my answers will either be helpful to the Avast team in fixing the problem, or give some hope to others who have had the same thing happen. As an inept computer operator, I do consider the system failure and having to restore to a previous state very uncomfortable situation and if sticking with the older Avast program prevents that from happening again, I'll be most grateful. Again, many thanks!