1. Check "winsxs
" folder inside your "Windows" folder. That keeps on growing in size as you install/update Windows components cuz it stores backups and other important things in there. You can check here
You can also use TuneUp Utilities to clean up your C a bit.
It helps you remove update backups.
2. Also, many apps and Windows itself stores temporary files on C: itself. Temp folders can be manually cleaned by typing "%temp%" into Run box and deleting everything you find in that folder.
3. System restore points are also primarily stored in C. They do take a considerable amount of space.
4. Another space eater is you Appdata folder located at C:\Users\<username>\ (system hidden). Applications use this place to store their temp files in specific folders. For example, Chrome stores ALL its data here in the Local folder, which can grow upto a gig.
5. All browsers store their cache and other data on C by default.
And BTW, what is that "installer" folder doing in there? That must be deleted if it's not your stuff.
These are probably the reasons why you see space on C vanishing day by day.
Hope that helped.