Windows XP, SP3
Since changing from Microsoft Security Essentials to Avast (Free), I’ve noticed that I can’t run ERUNT’s registry optimisation function (NTREGOPT).
I get an “Access Denied” error message but there is no flag or pop up from Avast and I can’t see anything in the (limited) log list.
It might be that something else is stopping the programme so I thought I’d check whether this is known behaviour for Avast.
Did you use the removal tool for MSE?
If not, I strongly suggest you do so.
http://www.ache.nl/index.php?location=mal-01
ERUNT:
It is a 10 years old tool and there is no development for it anymore.
I suggest you switch to CCleaner and/or Glary Utilities.
I don’t run the opt, but do run the backup - saved my butt more times than I can count but have you tried running under administrator?
Personally I would consider drive imaging software and run it once a week, or prior to making any major changes on your system. Keep at least the last 3 backup images and preferably more, I keep the last 6 - the imaging software also has a browser to view your backups and will even allow for extracting a single file or even restore a complete partition or drive.
That really does give you a robust backup and recovery strategy - far better and more complete than System Restore or ERUNT, etc.
If need be I would recommend the tweaking.com version
If it is only for a registry backup, no tool is needed.
Start > run > regedit > file > export
Thanks to all who replied.
I take all the points made re: registry backups and the the fact that ERUNT is an old programme etc.
What I want to know is why I had no notification that Avast was blocking it.
I got the “Access Denied” window but no indication that it was because of Avast.
Also, I placed the ERUNT folder in the exclusion zones but it made no difference.
Have now switched to Bitdefender and NTREGOPT runs without problems.
Your computer, Your choice.
Good luck. 
It is more satisfying to solve a problem then to simply run away from it. 
Running away only gets you other problems.
In this case less security.