If you are happy your AV is installed why not just tell security center you monitor your own stuff.
You mean stop all Services? That's not right
No, it isn't right.
You only stop the ones indicated in the step by step.Windows Management Instrumentation, and Security Centre.
If they aren't' stopped, you will not be able to delete the repository file. (Try it, if you want. Windows will say "Unable. In use by another...")
After the file has been deleted, the reason for the reboot is (1) Windows will rebuild the file, using up to date information, and (2) the services you stopped earlier will restart with Windows. All should be good.
If you are happy your AV is installed why not just tell security center you monitor your own stuff.
That's an option too. The security centre is not always a perfect warning system.
Personally, I like it present, in case something happens (hasn't yet) or I forget something (has happened) and there is almost always a warning produced.
So I treat it as a handy warning light, that can sometimes be wrong.
+ if you have something on your computer, and you're like me, you want it working correctly.
c:\windows\system32\wbem\repository
After following your full instructions I deleted the folder called Repository because you didn't specify any file within Repository. (It's still in my Recycle Bin.) I've rebooted and I still have the Security Center red warning on my taskbar. The 2 services you told me to turn off are running again after reboot which is normal. However the Repository folder within wbem has not rebuilt itself. Now there is no Repository folder within wbem. Should I put the one in the Recycle Bin back in wbem?
I know sometimes it can take a minute or two to rebuild itself. Try rebooting again and looking.
I did reboot again. Red symbol is still there. And what about that Repository folder still in my Recycle Bin?
Is this exactly how you did it Corona?
Disable the WMI service.
Delete the c:\windows\system32\wbem\repository.
Restart the WMI service.
Wait a few minutes until the repository folder has rebuilt itself (If you reboot immediately then the procedure may fail).
Reboot.
Okay, that's a little different than how I was told earlier. I'll give your instructions a try.
It looks like it's working now. Thanks Hazelnut!
...
Woops, spoke too soon. The red alert was gone after I did everything, but after I rebooted it came back up again.
No problem Corona, glad it worked for you.
Store the instructions away somwhere for when someone else needs them
edit...just seen your edit, back to the drawing board I guess.
See your pm's Corona.
If all else fails, and you are sure you are malware free, perhaps you could try uninstalling avast, rebooting twice (to remove drivers as rebooting once only disables drivers on security software) using the avast cleanup tool and reinstalling avast. Perhaps it may trigger the right response then as it seems like from what you say an avast update may have 'upset'' things a bit.
Assuming you are malware free, and the Avast services are actually running, the next step would be a full re-install from scratch.
Download the full Avast setup file (Direct download from Avast, 36.99Mb).
Download the Avast uninstall utility.
Disconnect from the internet.
Right click the Avast tray icon, select "Program settings>troubleshooting" and disable the self defence module.
Right click the tray icon and select "Stop on access protection".
Go to control panel>add/remove programs" and uninstall Avast.Reboot.
(You will loose any custom program settings you had, and anything that was in the quarantine will be deleted.)
Run the Avast uninstall utility. If there were any problems with the uninstall before (and you would know) you'll have to run it in safe mode. Reboot.
Install Avast from the setup file. Go through the setup prompts - if you are sure you are malware-free, you might prefer to skip the Boot scan, it can take a while.
Reconnect to the net, get updates.
I don't see how that is different to what I'd posted, except possibly for syntax. If something was unclear, please tell me, so I can change it next time.Okay, that's a little different than how I was told earlier. I'll give your instructions a try.
You know what? I got rid of K-lite Codec pack, uninstalled Avast, rebooted, installed Avast, rebooted with boot scan, and now it's okay. I remember reading a post somewhere (probably here) about codec packs causing problems. Nowadays I don't even think I need it anymore. I used to for the past few years, just in order to hear different online stations. But they all work now without K-Lite so, heh, bye K-Lite.
And Tarq, no problem, I probably did something out of order. I do that sometimes.
Tarq & Hazelnut, thanks for your help. 
Good news Corona, I'm sure Tarq will be glad to hear all is okay too
Yep, I'm glad all is well. Maybe K-lite was a factor. Don't know. And it probably doesn't matter.