Lastly, there is a hidden temp folder that can have a lot of files in it. Disk Cleanup does not delete the files in there. By default Windows hides system folders from you because if you accidentally delete them you can screw up your Windows operating system. Be very careful...
Click following for info to show hidden folders:
http://windows.microsoft.com/en-US/windows7/Show-hidden-files
Now to get to that hidden temp folder...
1. Click "Computer" icon to see the hard drives.
2. Double click the C: Drive.
3. Go into the User folder.
4. Double click the folder with your username / login name.
5. Go into the 1st folder on top called AppData.
6. double click on the Local folder
7. Double click the Temp folder
You likely have a lot of files in here especially if it has been a long time since you had your PC. When you use your PC Windows tends to store a lot of temp files here. Most are no longer being used and can be deleted, but a couple are currently active and cannot be deleted at all. To delete the unused / unwanted files...
1. Sort files by the date.
2. Select all files which does not have the current date.
3. Delete them.
4. Delete all folders as well that does not have a current date.
That's it you're done. I recommend you hid all system folders so that you do not accidentally screw anything up. I recommend you go through this particular process of deleting things in the hidden temp folder once every month or two months.