Want to get a DVD writer for my son's PC (Windows) but with all the talk about which technology is best (I understand Blu-ray is the 'accepted' standard - at least for commercial DVD's) is there something special I should look for in a writer?

Blu-ray may be the accepted standard for commercial high-definition movies, but certainly not for home computer dvd-writing. The accepted standard there is still ordinary DVD-R and DVD+R. Provided you get a drive that can write to both those types that's all you need worry about.
Blu-ray is way too early in it's life to become mainstream for home computers. For one thing, the blank discs are mega-expensive, and if you do put files or a movie on to one, you'd be hard pressed to find a friend's PC or dvd-player that can read it, so it could prove to be a waste of money buying into that standard just yet.

Sorry to but in on this thread - but I've got a bunch of .avi's that I'd like to be able to burn to a DVD, and watch them from my DVD player.

Is this possible without any additional software? I have a DVD burner and Nero Express, but all I've managed to do so far is to burn the .avi's as avi's.

They'll play on the pc via Media Player, but not on the DVD player.

Do they have to be burnt as an ISO image or did I make that up? If so - how do I do that?

Thanks in advance.

Blu ray is the new technology for High Definition viewing. The technology costs are expensive. Eventually it'll be mainstream but not yet. If you don't have a High Definition TV you don't need Blu Ray. Likewise if you have a car that runs on diesel you don't need to buy unleaded petrol.

Other poster - need convert the files to MPEG2 format, plenty of tutorials online.