In the original post gtafanboy specifically mention right clicking on a file to scan it with Avast, and I believe he is talking about the Windows Explorer right click or "context" menu scan.
I assume the scan icon/entry is appearing because he doesn't say that he can't find the option, just that the scanner doesn't "show up"
I have seen quite a few times when the scanning window or dialogue doesn't appear as such, it is actually behind the Explorer window, but it does bring up the scan icon or indicater on the task bar, and if you click on that you can get it to appear in front, or you can minimise the Explorer window and you will then see it.
This action is not consistent, sometimes the scan window comes up in front, sometimes you have to click on the task bar indicator, but either way the scan does work and the results are shown regardless of whether there is an infection or not.