Where is my disk space going?

I have Windows 7 Home Premium 64bit operating system with an Intel 80GB SSD where I have the Windows 7 operating system, and a 3TB Western Digital SATA drive for all of my non-program files.

The available disk space on my 80GB SSD keeps dwindling down to less than 1GB. When I go to cleanup the disk, I notice that there is only 49GB of space used up by all the files on my 80GB SSD (see attached screen shot), with about 6GB free. I have show hidden files and show operating system files turned on. I can't figure out what is taking up all of the rest of the space on my SSD. I realize that I won't be able to get 80GB of usable space, but when I originally installed Windows 7, I had about 75GB of usable space. However, it seems I only have about 55GB of space available now (space used by all of the files on C + free space). I have recently turned off hibernate, which allowed me to recoup the 6GB of space held by the hibernate system file on C:\, but my free space will likely dwindle down again, and I'm struggling to find files to delete so I have space available on my drive, which I shouldn't have to do because I should have 20-25GB of space available. Any idea where the rest of my space could be going?

I forgot to mention, I have cleaned out my recycle bin to make sure that it's not the culprit.

Check the size of your page file as well.

Control Panel > System > Advanced System Settings > Advanced > Settings > Advanced > Virtual Memory-Change

Look at the bottom of the dialog box at Currently Allocated

Looks like it's 8GB. Are you saying that 8GB would not show up in the total size of all files on the C drive, but would show as used capacity when I check the properties of the C drive? If so, that explains 8GB of the difference. Any idea about the rest?

I'm still checking my restore points. With regards to those, are you saying the space taken up on the HDD by the restore points will not show up when I view the total size of all files on the C drive, but will show up as used capacity when I check the properties of the C drive?

Ok, I checked my system restore settings (see below), and it's set to a max usage of 635MB, and is currently using 122MB. So I'm still left scratching my head as to where the space on my HDD is going.

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TreeSize Free - Quickly Scan Directory Sizes and Find Space Hogs

I appreciate the suggestions for space hog tools, but that's not the issue I'm looking to solve. I want to know why the cumulative size of all files on the hard drive (50 GB) plus the free space reported by windows for my drive (6GB) is less than my total available space on my drive (75GB). Or said differently, how do you explain the difference between the amount of used space reported by windows for my drive (68GB) and the total size of all files on the disk (50GB)?

When I go to cleanup the disk, I notice that there is only 49GB of space used up by all the files on my 80GB SSD

What tool/program do you use for the cleanup?
Can you post a screen shot of the "cleanup window" showing 49GB used?
You're not referring to the 2nd screen print in Post #1, right?
(That shows properties of the Recycle Bin)