If you had unprotected sex with someone who might have hiv.. are there preventative treatments/pills you can take to prevent hiv infection?

Get to your doctor: Unfortunately there is no 'preventive medicine' for hiv. If the person you were with did not disclose to you their HIV status, that is a legal matter and a very serious one at that. Get to your doctor asap and get tested regularly.

NPEP: This is what is called non-occupational post exposure prophylaxis, a combination of pills(tenofovir/emtricitabine/raltigravir) can be taken within 72 hours of exposure to the HIV virus for about a month ( if the virus is not resistant to this combination), this can decrease the chance of getting infected; however the best protection is abstinence or the use of a condom, see a doc for evaluation.