[QUOTE=3dman7;190675]1-delete all unnecessary files. move movies to external hard drives.[/quote]
Removing no longer used files makes sense, but moving movies to external HDs doesn't make any sense. You seem to be assuming that they are being stored on the C: drive, when they in fact be on other drives in the computer.
[quote]3-defrag your hard drive to free up more room.[/quote]
Defragging doesn't defy the laws of physics in that the amount of free space doesn't change. Defragging rearranges the files and empty sectors so that the files and free space are contiguous. Contiguous file sectors results in faster access.
[quote]4.Buy a bigger hard drive they are cheap these days.:)[/QUOTE]
Doesn't help with drive C:. unless you are suggesting that the OP rebuild his computer. Rebuilding M$ sucks in that all user programs must be reinstalled and configured. If the OP has a lot of programs, it becomes a PITA. There are some programs out there to help with that, but it still sucks.
None of this answers the OP's question. Why isn't 80GB enough?