Unzipping a .zip file: what's the latest?

This is embarrassing, but it's been so long since I used Windows that I do not know how to unzip a .zip file! My friend, who is below the level of computer illiterate, needs help installing a program in his Windows box, and I'm struggling to write an easy-to-follow HOWTO for him. (He's in California, I'm in Bangkok.) This guy does not want to learn anything about computers, and is really doing great to get and send e-mail. Anything more than that has to be spoon-fed in tiny bites and with tons of patience.

My HOWTO can assume nothing, absolutely nothing. So far it's been a real challenge for me, and when I came to the simple task of unzipping the downloaded software, I stalled. I don't know what the protocol is now, and I can't even remember what it was when I left Windows long ago. A Google just confused me, as there is a lot of outdated stuff on the 'net and some of it is proprietary.... It's a world that it really alien to me now.

I want to make sure he gets the easiest way to do it -- from the command line (I think you call it the DOS Prompt??) if at all possible. If we get into the GUI, I think we'll get bogged down and hopelessly lost.

I believe he has XP or the version immediately before it.

Suggestions will be appreciated.

TIA!!

If he's using XP or Vista, he should be able to just double-click it to open it.

That's great! Thanks very much.