Regarding the original question: if you have enough privileges (which is mostly equivalent to "if you are logged on as Administrator"), avast! uses a special way to bypass user restrictions, so yes, it might see files you normally don't see.
Note: there is no security problem here, avast! doesn't do anything you couldn't do yourself, even though some effort might be needed. The fact that you are an Administrator means that you have rights for (almost) everything. However, the access rights in Windows are slightly confusing... while you (as an Administrator) may not have rights to access a file, for example, you have rights to grant those rights to yourself