First you will need to get Virtual Dub v1.3c (the latest version with ASF support), which is a freeware linear Video Editor. At this time, the link to Virtual Dub's home page is here.

After loading Virtual Dub, select "File .- Open Video File" and choose your ASF file. After Virtual Dub quickly indexes the ASF file, make sure that the "Video" menu option is set to "Direct Stream Copy" and so does the "Audio" menu option.

Then all you need to do is "File .- Save AVI", and you will end up with the exact same file in AVI format, without any audio/video re-sampling.

Please note that Microsoft has intentionally botched their MPEG-4 codec to misbehave when played from AVI files. You can however, use the resulting AVI file in order to convert it to other formats.