If you are referring to right clicking on the .avi file and using the avast scan from there, then I have no problem doing that on any of my PC's, one with XP-Home SP2, one with XP-Home SP3 and 1 with XP-Pro SP3, all using Avast 4.8.1229 with current VPS updates.
When the scan starts it brings up the avast quick scanner dialogue. Keep in mind that .avi files generally range from big to enormous, particularly when the drives are NTFS format, depending on whether you are scanning them in the raw form, or after they have been cut up in an editing process (which will generally result in a larger number of smaller .avi files). Anyhow the point here is that it may appear to be not getting anywhere when the file is huge, and particularly if the PC is a bit slow, because the 0% scan progress does not increment in time with the scan, it goes straight from 0% to 100% at the end of the scan. Give it some time and you should find it will complete the scan.