I can't find out how my disc is filled. What is occupying space?

I have made a 15 GB partition of my hard disc of my dell laptop for windows XP. Now it tells me that there are only a few MBs of space left. I checked over and over again, after selecting the show hidden files option, but the sum of ALL the folders is a maximum of 7 GBs. I checked every folder and did not even miss a single one. The biggest ones are two folders, approx 3 GBs each. My program files is 1/2 GB and the rest are all a few MBs lesser than the former 3 folders. I have tried disc clean up, tried adding additional files everything. I selected ALL files AFTER making all the hidden files shown and it shows me 7.8 GB but when i check the properties of my partition, it says 14.9 GB