Low disk space on Lenovo D:/ drive?

My computer keeps coming up with a message saying that there's low disk space on Lenovo D:/ drive and sure enough when I look at drive there's only 1.58 MB left on the 28.9 GB drive but when I actually go into the drive the is only 1.74 GB actually used. Does anyone know what has happened to the other 27.4 GB and how I get it back?
Any help would be appreciated.

Your HDD may be partitioned, if your on Windows - go into computer and if you see two HDD listed. Your main HDD may be partitioned which means your HDD is split into 2. So windows mainly uses the C drive unless you have changed it. To sovle this you can merge the two by using this software: http://download.cnet.com/...63346.html

Be careful though. Sometimes it can mess up.