A handy tool called WinDirStat (Windows Directory Statistics) can be used to easily find which files and folders are taking up the most space on your drive. From there, you can delete them and open up a ton of storage space.
WinDirStat is a disk usage statistics viewer and cleanup tool for various versions of Microsoft Windows.
Note: If you are looking for an alternative for Linux, you are looking for KDirStat and for MacOS X it would be Disk Inventory X or GrandPerspective