Download the latest Captivate USB Drivers directly from here:
http://www.samsung.com/us/consumer/mobile/mobile-phones/at-t-phones/SGH-I897ZKAATT/index.idx?pagetype=prd_detail&tab=support



For some reason if those don't work, try the general Samsung Galaxy-S drivers here:
-64-bit OS (http://db.tt/3yiHg7)
-32-bit OS (http://db.tt/hLxILM)