|Before messing with the solid block of immovable files, make sure that it is not your Recovery Partition that you are looking at.|
As mentioned already, deleting or moving files to another external drive are the only ways to make more space on the drive.
If you have programs that you never use, you can uninstall them and this will help.
If you want a temporary increase of a few GB, you can reduce the amount of space used by System Restore from the default 12% to around 8%. Doing this will simply make available less restore points but it is not advisable to go much less than that.
About the only other option you have is to purchase a larger hard drive for your laptop and clone the original drive onto the new larger one. Depending on what software you use to do the cloning, you may need to expand the copied partition on the new drive or add another partition and format it in order to make use of the entire drive.
You have to be a little bit crazy to keep you from going insane.