Well in that case, I don't know what order you built in, but might I recommend one? If so, the following is a very solid choice on all AD carries:
Boots +3 pots to start
(Optional: up to 2 or 3 Doran's Blades)
If no Doran's Blades, Vampiric Scepter
Phantom Dancer #2
Turn Vampiric Scepter into Bloodthirster
Last Whisper if they have a lot of armor. Take this earlier if you see them building a lot
Guardian Angel is also a fun last item
In general this will give you maximum damage per second and sustain with the lifesteal. Hopefully this will be subject to change when they start adding items in season 3, but for now this is awesome.
Boots +3 pots
BF Sword on first back (1-2 Doran's Blades if I go back before I have enough gold for BF Sword)
Finish Infinity Edge
Finish Blood Thirster
Finish Phantom Dancer
Get Guardian Angel if your dying at all in team fights
Get Last Whisper if they have any armor
That is my build order in every game when I play ADC. I usually don't go for 2 or more Phantom Dancers but if you have a champion like Vayne it may be a good idea, you will then want to go for a Guardian Angel so then you have some defense in the team fights. I personally wouldn't get Last Whisper on Vayne though because of her True damage in Silver Bolts.
So when building, most items can be situational, but some of the items like Infinity Edge and Blood Thirster are staple to any AD carry.