How I could extend my system partition for 2011 free???

Hello, everyone, first time here and I hope you could give me some suggestions because I am really confused for a long time. My server 2011 system partition is running out of space and I am going to mad because essential applications cannot be installed in any longer. I must admit that I was stupid; I just leave 20GB for system partition when I install the operating system. Does anyone could provide me some suggestion or solution? I do appreciate!