I recently installed one Vista version (Ultimate) over another (Business). I originally tried to upgrade from Business to Ultimate, but got a blue screen upon boot after the upgrade. So I did a new install of Ultimate. As you might be familiar with, the installer will put your old Windows installation in a folder at C:\Windows.old\. It does this whether installing over XP or Vista.
How this relates to Steam is that I had Steam installed on my old Vista, and had quite a lot of content downloaded (17.5GB). Unfortunately I did not use the backup feature of Steam to backup all this to a file before I installed Vista again.
Needless to say, I didn't really relish downloading that much stuff again (it's probably 10GB when compressed). So I installed the Steam app from here, and then went digging in Program Files. It looked like most of the data was in the folder "steamapps," so I copied that from C:\Windows.old\Program Files\Steam\ to C:\Program Files\Steam\. One file was a duplicate between the two, and I DIDN'T replace that one. Then I started up Steam, with my fingers crossed. It showed all the games and trailers still there, but the games needed updating. They each took less than 20 seconds to update, and I was off on my merry way.
So the short of it is that you can just copy the steamapps folder from one computer to another (probably you have to have the same Steam username on both computers though). I haven't tested all the games yet, but Counter Strike: Source and Half-Life 2: Deathmatch do work, and all the other things I have are based off the Source engine anyways so will probably work. I'll update this if I ever find a game that didn't make it through the transfer.