Check out Dr. Dick's article on this oh-so-common STD:
If you're HIV-negative, and think you might have come in contact with HIV (condom break, condomless sex, needle stick, etc.), you might consider getting on PEP (Post-Exposure Prophylaxis). PEP can lower your chances of getting infected with HIV. How does it work? After a possible exposure to HIV, you take a 28-day course of HIV medication to decrease the chance that the virus will settle in your body.
Read on for more info: Feelin' Peppy