I had an issue with Steam recently, probably unrelated to yours but thought it might be nice to share in case anyone else ever encounters this.
I have Windows installed on an SSD and everything else on a secondary hard drive. This way it's easier when I format Windows and re-install everything. Usually when I format my OS drive, I just leave Steam where it's at, then re-install the Steam client once I get Windows up and running again.
I've never had an issue doing this until my latest re-format a couple weeks ago. I tried to update Mass Effect 1, but it wouldn't do it because it couldn't find the CD key. I couldn't install any of the DLCs either for the same reason. I had launched the game and played it, but it didn't create the right registry keys like it was supposed to. I ended up doing it manually and thought all was well. Then I tried to launch Fallout: NV, but it wouldn't detect the game. The only option I had in the launcher was "Install", which obviously didn't work because it's the Steam version.
I did some digging on the Steam forums and ran across someone that suggested picking the "Check for Steam client updates..." option in the client. I did that, Steam said everything was up to date, but a few seconds later it came back and said that the Steam Client Service wasn't running. I told it to install it (took a while, about 15 minutes) and everything has been great since then. I'm guessing the service gives Steam permission to do the things it needs to do in Windows.
So something to be aware of - I've never even noticed that windows service being there.