Assuming you are using authentic Chromium snapshot builds, can you elaborate more than "Can't install"? Do you see an error message?
And yes, it sounds like something is fishy with your Windows. Though you are possibly confusing the terms. As far as I know (I am not a Windows 10 user), the default applications screen lets you pick a browser to set as your default browser, it does not let you install a browser.
Also, Avast is not a browser.
And I am not sure what "pop-up browsers" are. :)
By the way, Chromium is not meant for normal browsing, it lacks some features that Google Chrome includes and it does not update automatically, which significantly increases the risk of being hacked risk over time.