Missing hard drive space after upgrading to Windows 7

Good afternoon all, first, thanks in advance for any help you guys can give. I recently upgraded my 10 year old custom built PC to Windows 7 and now I am missing roughly 100gigs of space from my hard drive. I am attaching the two pictures, one is the expanded disk management screen and the other is a properties look at my C drive and the top 4 folders on the C drive (by far).

The top folders consume about 110 gigs of space and the listed used space is 200 gigs (the next 3 largest folders don't add up to 5 gigs combined). I also don't show a hidden partition from the old OS in the disk management screen, so I have no idea where that 90 gigs went.

I know with the age of the system it's possible the HD is dying, but my HD test didn't come up with any errors. I've also gone through defrag and system cleanups.

Any ideas of where to look would be greatly appreciated. Thank you!

Case

Thank you for the quick responses folks you're awesome! Wow that was interesting. First to answer your questions ignatzatsonic: 4 gigs of ram, and I'm not savvy enough to know how to check for system restore, hibernation or page file settings. Now on with the show:

According to WinDirStat my HD was only 130 gigs in size (which is about what everthing else adds up to), so I tried TreeSize Free and got the first part of my answer immediately, so thanks for recommending that app. I've attached a few pictures of both apps. The answer to my issue appears to be a folder called "System Volume Information" which I'm guessing is a part of my system restore. According to TreeSize, it's eating up 70 gigs of space which explains almost all of the missing space (the rest I'm sure is just interspersed throughout a million different locations). Now If I had to further guess, I would say that when I did the install, which was an upgrade (not clean), even though I tried to delete all the old stuff such as windows.old I didn't delete the old restores. So I'm guessing 10 years of system backups takes up a lot of space!

So the question now is, how do I clean this up? I'm not sure any of the restore data is important to be honest but I leave that to more experienced people. Your continued assistance is very very appreciated, thanks folks!

That did the trick, thank you so much. I would have never figured that out via my usual troubleshooting methods (IE Google).

Attached is the end (and accurate) result.

I can honestly say that in 10 years of owning this PC I've never used the system restore except once, and I can't even remember why it was soooooo long ago. I do use Avast though.

Thank you for the advice, is there an alternative free anti-virus that you recommend?

To tame Avast:
- go to control panel-> programs and features -> Avast -> change
- remove the component NG (may also be called-Secure Virtual Machines or Sandbox).
- change system restore % back to 10% (windows default) or whatever you like it
- restart
- check if % is what you set
- check restore points (seems this NG delete old restore points when run out of space)
- manually create a restore point if there are none
- open command prompt ->vssadmin list shadowstorage
- see how much space it takes.