This does sound like conflict, as avast doesn't block, but scans and alerts to infection, which presumably you don't get or you would have said.
Why would avast single out MS applications other than all applications, that just doesn't make sense. MS Office applications have macro capabilities in their documents and would be scanned, certain AVs have hooks into the MS Docs and even when uninstalled. These hooks remain and this can cause conflict, which appears to be blocking as the hook locks the file.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
Do other browsers work, e.g. firefox, chrome or opera ?