Steam is Valve's game content delivery system. You need it to play any of their games. It has to be connected to the internet.
My steam install went bad. I deleted the clientreg.blob file to force a re-download. That was 9 hrs ago... After 1/2 a dozen attempts the best I've managed to get is 28% before it all goes tits up.
Currently at 5%. I've asked for help on the forums but at the moment nothing useful to help. This really sucks, I have 3/4 games all bought and paid for that I now cannot play or access (HL2, CS:S, DOD and DOD:S). It's all fine and dandy when it works, but when it does not you're screwed and there's f*ck all you can do about it...
Forum Post Here
Update: found out that carphonewarehouse (TalkTalk) fair usage policy (P2P bandwidth shaping) is breaking STEAM and WOW