Please post what memory is installed in each slot (you can use CPU-Z to get that info), how much memory you see in Vista and what you expected. Something doesn't make sense here.
Based on your last reply, it looks like you have two 512MB DDR2 667MHz modules, one 1GB DDR2 667MHz module and one 2GB DDR2 800MHz module. I'm not sure your motherboard will support such a configuration, but it probably won't work in dual-channel mode.