Windows Vista 32bit does not recognize all my memory?

I have a Acer 5520 that was running the factory image vista home 32 bit. I don't like all the junk with a factory image and the laptop had came with a vista home cd so I installed it and now it says I only have 767mb of memory when it should say 1gb. Before I took out the factory image it had said 1gb. Any idea as to why it is doing this. Thanks