I had the same problem- have a 256GB Samsung 840 pro with Windows 7 and less than 30GB of other stuff, but I was down to 74GB remaining for no obvious reason. I did all the tweaks, like reducing the page file size, running disk cleanup, clearing caches, yadda yadda yadda- and regained a measly 3GB. Obviously that wasn't the issue. Luckily, I found the problem (for me):
I had a sneaking suspicion that Windows Updates were the culprits, and it turned out it was. Turns out that Update stores all the "Important" AND "Optional" updates (even the old ones) in a file, and they stay there, whether you choose to install them or not. This was eating up tons of space, and here's how to safely clear them out and do a little prevention trick at the same time. The best part is, it's easy!
STEP 1
In case noobs are reading this, I'm going to describe every step:
Left click on Windows button.
Left click on Computer
Left click on C drive
Left click on Windows folder
Left click on Softwaredistribution
RIGHT click on Download
Delete it
Restart your computer
Now check your C drive and see how much space you regained... I literally reclaimed- I kid you not- 102GB of space! Now I have 181GB of free space, which sounds about right.
STEP 2
Future prevention:
Open Windows Update in control panel
Click on Change Settings
Open the pull-down menu under Important Updates
Select "Check for updates, but let me choose whether to download and install them"
Press OK at bottom of screen
Done!
Now your pc won't download any updates until you tell it to, and this problem shouldn't happen again. If it does, just repeat step one.
I hope this helps you or anyone else. Cheers!