did you change the entire headlight housing? or did you change just the bulbs?

first check to make sure you attached all your bulbs properly. an unsecure bulb with a worn weather strip will let moisture into your housings. if those are completely sealed up, and you did change your complete housing, you'll have to reseal them using silicone.

if you have HID's right now, i'd remove them. condensation is the #1 killer of bulbs.