I remember that used to happen to me. Problem could be one of two things that I remember: first, try completely uninstalling Steam (even delete its folder) and then reinstalling it again. You can move the SteamApps folder before you delete it so that you can move it back and won't have to download all of your games again. Second, I remember when this used to happen to me, my computer was HEAVILY affected by a virus called
PE.PARITE. Before you uninstall Steam, check for this virus. This virus also crashes Steam with a sudden "Floating-point Exception" error. If you don't have the virus, just try and reinstall Steam as explained above. If you do have it, send me a PM.