|If we knew the size of the hard drive, the sizes of each partition, and what you have on your C drive (OS, programs, files, etc.), then we can make better recommendations.|
In general, these are thing that might help some, depending on what is going on there.:
Uninstall programs you do not use.
Move personal files to another drive.
Run Disk Clean Up on your C drive.
Clear out your browser's History, Cookies, Temp Internet Files, etc.
Delete old saved emails and then clean out the deleted items folder in there as well.
Check your virtual memory settings, they are typically 1 1/2 times the amount of memory you have (in MB), more you rarely need, and if you have 8GB or more of physical memory, you can use the same for your virtual memory (1X).
Check your System Restore settings, They should be around 15% of your disk size, but you can safely use 10% or 12% as the maximum setting.
Make sure that any back up system you may have is sending the back ups to another physical hard drive or external drive, it does you little good and wastes space if it is using the same drive.
Run Malwarebytes to make sure that you do not have malware clogging up your machine.
You have to be a little bit crazy to keep you from going insane.