Windows uses memory dynamically and will make use some of your free memory if it's not otherwise required. I've installed Windows on systems of various sizes, and the amount of memory it uses varies - on a 768MB system it takes around 400-500MB, while on a 12GB system it takes around 1.5 - 2GB. This is a good thing, it means you're getting value for the memory you purchased. It's a bit of a waste to pay money for memory that just sits empty all the time.
If you load up the system with application programs that actively require memory, Windows will automatically release what it doesn't need to optimize performance.