Microsoft has a knowledge base article on the error.

Using the second part of the article may be easier - using the last known good configuration.

Here is a link that might be useful: MS KB Article

After several tries, I was able to boot in safe mode and ran a system restore. Now my Avast won't work.
I click on "fix now" and "start Program" and nothing happens.
When I try to turn on the real time shields, I get this message..
Unable to start the File System Shield! Shield is unreachable. Looks like I will have to reinstall Avast.... Updating Avast is what started this mess.....

{{gwi:1487207}}

I had to many problems with Avast, the last wouldn't allow me to play the games I have play for a very long time. I changed to MS Security Essentials and have not had one problem with it. It automatically updates and scans. I don't know if it will work with XP.

Microsoft Security Essentials does work with XP but you need to have Service Pack 3 installed first.

I've been working on this most of the morning and fiddled around and was able to go to the last know working configuration and Avast is now working. I have been having boot up problems for quite a while, but have been able to do a work around.

Thanks for all your responses.

Jean