Open "computer". Right click on drive C: and select "properties". Then check the box labeled "compress data to save disk space" and push "apply". It could take a while to process, but when it's done you should have more space available. Compression comes with a downside, though. Although it decreases the size of data being used, the CPU has to decompress it every time it wants to access the data, and that takes time, so compression might slow data read/write times a bit.
Or backup your computer and go on a deleting spree!