Call of Duty Black Ops PC Zombies hasn't worked for a week now

Call of Duty: Black Ops' Zombies mode has been unplayable on PC for a week now, leading to a glut of negative reviews on Steam. Last week, players noticed a server error message pop-up when trying to access Zombies from Black Ops' main menu. Zombies is a co-op focused mode, but this issue prevents players from playing solo, too. Here's how...