xp won’t recognize added memory

I have a asus MB A8N-SLI Deluxe with duel channels, with two kingston 512 sticks and two 1GB sticks in the other channel, not kingston but same latency, BIOS recognize the add memory but windows xp don’t. I have the two 1GB’s in the first channel and the two 512’s in the second two channel can anyone help me??