Most of the disk space is
unexpectedly disappearing into one or more of several places - like recycle bin and system restore storage. Already mentioned above is pagefile/swapfile, and another is hibernation file. Occasionally system dumps take up space.
Recycle bin - you might be familiar with that - it's where your deleted files go, and it is quite easy to empty. Right-click on the recycle bin icon and select Empty recycle bin.
Pagefile - used as overflow for your PC real memory. Typically 1.5 x the size of real memory, but that is stupid if your real memory is 4Gb or more. You might prefer a smallish and fixed pagefile in that case. See
http://www.tomshardware.co.uk/forum/16989-63-what-size-give-pagefile
Hiberfil.sys - used as a place to store the entire memory in current use when your PC goes into hibernation. Foolishly, once the hibernation file is created, it takes up nearly as much space as your real memory, and is never deleted again until you do so yourself, and then disable hibernation. You dont see this unless you give yourself rights to see hidden and system files in explorer.
http://www.howtogeek.com/howto/15140/what-is-hiberfil.sys-and-how-do-I-delete-it/
System restore - this is the one that does not get included in disk space totals. It is controllable by adjusting the percentage of disk space you are willing to devote to it. Here is a link to someone I previously advised on this topic
https://www.techspot.com/community/topics/where-does-my-space-on-the-system-drive-go.165532/ see post #5
System error dumps - if you are in a habit of running poorly written software, your system might often crash, and when it does, a dump will be taken, which can be controlled as to whether it is a full memory dump (there goes 4Gb !) or a minimal dump. These dumps are very rarely of any use to anyone, so
http://www.tomshardware.co.uk/forum/265378-45-memory-dump-files
Log files - much software involved in PC communications and protection likes to log it's activity, but never draws the users attention to what it is doing (eating up large amounts of drive space).
https://www.techspot.com/community/topics/where-does-my-space-on-the-system-drive-go.165532/ again, this time posts 10 and 11