[doveman2]

The Pi will play MPEG-4 compliant video. From http://en.wikipedia.org/wiki/Windows_Media_Video:

Microsoft MPEG-4 version 1 MPG4 Video for Windows-based codec. Non-standard MPEG-4 codec incompatible with the later standardized version of MPEG-4 Part 2.
Microsoft MPEG-4 version 2 MP42 VfW-based codec. Non-compliant with finalized MPEG-4 part 2 standard.
Microsoft MPEG-4 version 3 MP43 VfW-based codec. Non-compliant with finalized MPEG-4 part 2 standard. Eventually locked for encoding only with ASF files (build 3688 and earlier could also encode to AVI).

so, no, these are not MPEG-4 compliant and can't be played on the Pi.
handbrake is quite simple to use to convert them.

In future make sure videos are encoded with open standards (H.264 is the obvious choice now) not proprietry variations.

Oh, if only life were that simple!. My brother gets a lot of clips from downloads or sent to him I guess, so he's not responsible for and can't control how they're encoded. With his old PC, he had that hooked up to the TV and could play all these clips fine but now with his RPi he can't play half of them, so it doesn't seem like progress to him and it does seem rather sucky that the RPI can't play them. Is this because they use a proprietary codec that isn't available for the RPi or is there some fundamental technical hardware reason they can't be decoded or could in theory a codec be available in future that would decode them on the RPi?

I guess he'll just have to convert them for now but is there any way to transcode them to be compliant without degrading the quality further or will they have to be fully converted. I'm not sure if Handbrake is simple enough for my brother and it seems he'd have to specify a Destination file for each one, which will be a bit tedious if he's got a lot to convert, so something that will just let him add files and automatically create the output files in the same folder with an added suffix will probably be a lot quicker for him, if there's something like that.