System Restore keeps changing to use 50% of HD

This is weird. A few months ago I had reason to suspect I had a virus, and went through a thorough process to make sure my system was clean. One of the symptoms I noticed was that I seemed to be losing disk space. Well, after my system was clean I did some further research and learned that System Restore was the culprit. I had not, as far as I can remember, set the configuration for System Restore. I just assumed that guys who built my box and installed Windows 7 had done it correctly. When I went to the configuration window I discovered it was set to use 50% of my HD. Ridiculous. So I reset it to 7%, or about 20 GB. OK for a while, and then the same problem, my drive free space was suddenly much less than I knew it should be. Went back to SR configure and it was set to use 50% of HD again (well, actually 50% of the C: partition, I have the drive divided into three partitions). Changed it back to 7%. Now today I had to do the same thing. Any ideas as to why Windows keeps resetting this? I know for certain it is not something I did, but I'm also confident there's no common virus or malware on my system, so it's very puzzling.

Some people have a problem where Avast keeps setting SR to 50%.
They released a new version a few days ago.
You could try the new version and see if it fixes the problem.
https://forum.avast.com/index.php?topic=167184.0

Ah, thanks. I checked Avast! and I now have the latest version (the one referred to in their forum). It was installed at least three days ago, though, so the problem might lie elsewhere. I'll have to wait a couple of days and see if it happens again. I don't know of any way I could tell when the change happened this time.

If you have Avast NG installed you can try removing that and see it fixes the problem.
You can read more about this problem here:
https://forum.avast.com/index.php?topic=162041.0
https://forum.avast.com/index.php?topic=163039.0

Well, I didn't have NG, whatever that is, but another suggestion on the threads cited was to disable Grimefighter. Didn't know I could do that, glad I can. The only thing it does for me is once a month or so tell me I have two unnecessary programs and 13 configuration mistakes that make my computer slow, but if I want to find out what they are I have to upgrade to a paid version. Hope it stops, but after I rebooted the slider was back up to 25%. I would hate to uninstall Avast!, but I used to use AVG and wouldn't mind going back to it, or even Clam AV.

I have Avast free on an OS and haven't had this issue.
I don't have any of the optional Avast "bloat" installed, i just use the basic protection.
I have MS SR turned off, I use Macrium and Acronis System Backup Images.