The only solution is to replace the entire motherboard. If it's under warranty, call it in, otherwise you're basically screwed.
My second memory slot is not recognizing the second memory stick. I have tried the original 512mb stick (in pairs) and two new 1 gb sticks (in pairs). The system has Vista and an AMD Athlon 6400 processor. The original memory is 1024mb DDR2 dual channel memory 667MHz