Steam users will be prompted with an update today which will update their Steam client with the latest addition to the widely used service. This update will contain the Steam Guard system which will not only protect your Steam account from hackers but also notify you if anyone has even tried to get into your account. Gabe Newell himself gave his password and username out at GDC in an effort to prove how resilient Steam Guard is.
As long as users have a verified email account attached to the account they will be able to restart Steam and it will be locked down on that computer. Any time someone tries to log in on a separate computer an e-mail will be sent to that verified account to notify you and also provide a code in case it was you yourself trying to access your account on a separate computer. Meaning that if your old computer breaks down and you get a new one, your Steam account is completely recoverable.