Get Spacemonger:
http://www.sixty-five.cc/sm/v1x.php (no need for an installation, just start the executable)
This tool will create a graphical representation of your hard drive files and folders in proportion to the actual space they occupy. You will see at one glance where the largest files are, and you can right-click and delete them from inside Spacemonger.
You are especially looking for temporary files, large log files (*.log), backups and large temporary sync files; but there may be other single files that have an abnormal size. If in doubt, google the file names and/or exctensions or post here.
Also run CCleaner:
www.ccleaner.com as a start to get rid of all normally known dispensable files before inspecting the drive with Spacemonger.