I am thinking you have no room to run even the operating system your startup disk is so full. I am not sure by your question if you are running a Mac or Windows computer. Either way, you MAY be able to use your original disc (DVD) that came with your computer and boot from it. This way you are not using the hard drive to boot to. Then you can either wipe out your hard disk and reinstall the original Operating System or there may be some way to delete some of the data or programs and restart your system up afterward. If that is not possible, you may want to either replace your hard drive with a larger one and re-install your operating system from your original disc. Then re-install only the stuff you need and not overload your hard drive again. Another option is to remove your hard drive, purchase a disk transfer cable, hook it up to another computer then delete some of the junk off of it so your operating system can have room to load.
If you go the route of putting in a larger hard drive, you can re-install your programs and have more room. This is one of the bad results of overloading your hard drive. If you're tech savvy, this is easy; if not you may want someone else to do one of the above methods for you.