If you're using NTFS file system with Windows NT 3.51 or higher, you can easily compress individual files or full directories to increase the capacity of your hard drive.
For example, if you want to compress all the files inside a directory (including sub directories): go to a "Command Prompt," change to the directory you want to compress, and type:
COMACT /C /S
To see which files and directories has been compressed so far, type:
COMPACT
Compressed files will be decompressed on the fly by Windows as necessary. But, if you want to remove compression for a particular file altogether (for example, to reduce the processing time required to decompress certain files) type:
COMPACT /U