You have a WinDirStat graph, which is a great start.
Let's first deal with the obvious one: looking at your WinDirStat graph, you have 7.4 GB in the recycle bin. Empty the recycle bin to free up that space.
Your Windows directory is the largest, but 30-40 GB is pretty normal in my experience. The "installer" and "WinSxS" folders under your Windows folder (which you mentioned in a comment on another answer) contain backup copies of the installers for any software or Windows features/patches you've installed. Microsoft has an article describing how to safely reduce the size of the WinSxS folder but, as Michael Frank mentioned in his answer, you won't be able to uninstall those service packs, updates, or components later. Don't even think about clearing out the "installer" folder, because doing so will similarly prevent you from uninstalling many applications including MS Office and many third-party software products.
The 16 GB pagefile.sys is the Windows swapfile. Rebooting will clear it, but it will regrow automatically as-needed. You can set a fixed size or move it to a different drive in the System control panel, but messing with the swapfile can have performance implications.
Next up is your Users directory. If you log into each user account and enter %TEMP% into Windows Explorer, you may find a large number of temporary files that can be deleted. Preferably, you should delete these via the Disk Cleanup applet, but you can delete them manually, too. You will probably also find a bunch of crap in each user's Downloads and Desktop folders. Delete unneeded downloads as appropriate.
The Program Files (x86) and Program Files directories are where your installed applications live. You can uninstall applications from the control panel to free up space in these directories, assuming you didn't delete the installer backups under the Windows directory.
ProgramData is where various applications store data that should be shared across multiple users on the same machine. Some applications actually install to this directory so they can auto-update themselves without requiring administrator permissions.
$Windows.~BT is where Microsoft caches updates, including the new Windows 10 upgrade. If you've reserved Windows 10, don't delete this directory until after you've installed the upgrade.
Unfortunately, 120 GB is a pretty paltry amount of storage in a day of 10+ megapixel photos, HD video, and 20+ GB games with hi-res, immersive graphics. If you play games or take pictures or record video, you're bound to spend a lot of time just trying to free up space on your hard drive. This is probably the last thing you want to hear, but you'll be much happier in the long run if you upgrade to a larger SSD. Fortunately, they're much cheaper and commonly available in much higher capacities than they were only a few years ago.