I'm having problems previewing video avi files using realplayer. Each time i try a message comes up saying 'RealPlayer needs to download new software to play this clip' then when i press ok it searches then tells me: 'there is no software update available from Real to support this content.' the details say that the content type is ICM.XVID.
Should I be using a different media player? If so which one and how would I change the default?
Will these files start working when they are fully downloaded?

It means you don't have the appropriate codecs to play the file. I'd recommend getting VLC - (click on link) _ (choose nearest mirror site) as it's very reliable & has in-built codecs.

Thanks! I'll give it a go!

I downloaded the videolan player but nothing happens when I try to play the video. The videos still have a realplayer icon next to them?

You'll need to change VLC to become the default to play them. See Setting Default video app