avast has no control over the files other than avast, so I have ni idea what else is going on. What I do know is if it is an avast file it won't let go (other than for the avasy4.ini file, my image2) it wouldn't let be changed (my image1) and the only option was to exit abandoning the changes or saving it to a different file name.
I just had a play with the msconfig and it lets me change startup entries it even let me uncheck the ashDisp.exe entry and apply it. At this point you get an image (about restart, my image), I choose not to restart. I opened msconfig again and the change I had made to one non-avast startup entry was still as I changed it, but doing the same with ashDisp.exe had reverted to its previous state. Now during all that I never one got a permissions error or an avast self-defence alert, it just didn't let me change ashDisp.exe.
So I'm unsure what is going on in your system, but msconfig is acting as expected for me.