Open Question: Missing GB on windows 7.?

I have win7 os 64bit and my computer is 250gb big and it says 25.8gb free of 221 i know those 29gb are processing from the 250gb but if i add up all the gb i am useing im still missing 42gb how can i find these or get them back? I heard when you delete stuff it keeps the files on your computer with very small memory so it can always be rocoverd how would i delete those if thats the problem? i have check every folder and windows old folder and cant find these 42gb.