I'm told that TechNet would allow me to install operating systems on client's computers, provided that they have a valid product key, without needing CDs/DVDs.
You don't need technet for that. But it makes it easier to obtain clean install media. As long as they have a valid COA, or Product Key, it doesn't particularly matter where the media came from.
If you have a windows 7 DVD, create an ISO file of the DVD. Then delete the ei.cfg file from the sources folder. When you burn the modified ISO to disk or create a bootable thumb drive with it, instead of being locked into the one version you get the option to pick from all the versions. Your product code has to match the version you install so you can't cheat. You can't install Ultimate and enter a Pro product code, it will be refused. I did one for 32 and 64 bit each on a bootable thumb drive. I used this to create the thumb drives. http://store.microsoft.com/help/iso-tool
Don't know about Vista, but for XP each version is separate media. Home, Pro, Pro VL and MSE. I've seen all in ones that somebody has made offered up on Bit-Torrent sites but I won't go near that stuff. You don't know what bonus extras are included, if you know what I mean. A retail key won't work on OEM media and vise versa. For 2k I think there is only Pro (workstation) and 2 or 3 server versions. You can't download the ISO files from Microsoft without a Technet or MSDN subscription > $$$$ So the best you can do is ask around and see what you friends are running. Then see if they will burn you a copy of their media. If no product codes are exchanged it shouldn't be a problem.
You could use your Technet media to install on a client PC as long as you use the product code from the COA sticker on the client PC. Your MSDN or Technet product codes are for you and you alone, but I think you already know that..