I'm a little sad about this but I have to say that avast + windows10 is a no go at the moment. Both desktop and a laptop show the exact symptoms and it's not just me a friend of mine also says the same... so yeah good luck avast, hopefully you guys aknowledge these problems soon.
There is no incompatibility between Avast Free and Windows 10. You may have an individual computer hardware or software problem on the systems you describe that need to be sorted out.
The incompatibility isn't between Windows 10 and Avast. In my house alone, I have 6 computers all running various versions of Windows 10. All protected by Avast free. Some computers are fairly new,
some of them are dinosaurs from Windows XP vintage. All are running without problems.
I suggest you follow Asyn's suggestion and use the Support Tool to get to the bottom of your problems.