try using treesize, that should tell you where all your space is used.
Some things that come to mind, check if you have Hibernation enabled. If it is, you'll have a file that that uses as much gbs as ram in your computer. To do this, open a CMD prompt with admin priviledges (right click - Open as admin) and type the following:
Code:
powercfg.exe /hibernate off
Another one you might want to try is disabling the pagefile, which will give you a couple gb's back. To do that, you'll have to:
Right-click Computer
Select Properties
Select Advanced system settings
Select the Advanced tab and then the Performance radio button
Select the Change box under Virtual memory
Un-check Automatically manage paging file size for all drives
Select No paging file, and click the Set button
Select OK to allow and restart.
Or, you know, you could save up a couple bucks and buy a 10-years-old laptop that will probable come with a 120gb hdd