Lost space on hard disk

[Vadik]

I have about 51Gigs of total space on C drive. Total space occupied is about 15Gigs, but it shows me that there are about 10Gigs of free space left. My question is: "Where is the other 25Gigs of somewhere lost space?" I checked everything, but could not find it.
Thank you in advance

 

[Kharn]

Ok this is your primary drive i am assuming, before we can go further what operating system are you running XP or Vista an have you formated or done any partitioning to the disk recently?