My computer hard disk size is 24 4 gb

Disk Cleanup like CCleaner to wipe away all the temp files, recycle bins, cache stuff etc... Have a small OS installed. Linux isn't that big. Uninstall windows games, and unnecessary programs.

My number 1 suggestion is to get a bigger hard drive. 24.4 GB is extremely small and if you're running any version of windows, you're out of luck.

Turn off system restore.

Wait a minute your hard drive is only 24.4 GB? Time for a new Drive.

Uninstall programs you do not use. Do not just delete program files. Make sure you uninstall properly. After that, use CCleaner as moornix has stated, delete videos or files you never use or watch.