Hard drive won't shrink

Hi there, I was hoping somebody could help me here! I have two WD scorpio blue 500GB hard drives in my laptop and both have ~150GB free. I wanted to shrink the C drive (it doesn't have any other partitions, just the main one). So it has a capacity of 465GB and 154GB free. So, I'd like to shrink it, however, the windows disk manager will only let me shrink it a very small amount. I have a...