The fact that it's in PATH doesn't explain why it's loaded there... the modules are loaded by name and avast! doesn't use msvcr80.dll at all (it uses msvcr71.dll).
I haven't been following MikTeX lately, but I wouldn't expect it to load into other processes... it's mostly a bunch of console apps

Is there any reference to this particular path (miktex\bin\msvcr80.dll) in registry?
Is there actually any reference to miktex path in registry (except for the main miktex key, and maybe associations like Yap)?