Space missing on my hardrive

Hi

I ve just have a look on the space remaining on my hard drive,there 200gb free of 600gb on the HD, but when i check out the space taken by the files on drive C I came up with a total of 150gigs,there's about 350gb missing an i can't figure out how to recovedr them.The only thing I can think of, when I was having troubles installing windows the first time.i peformed the operation several times before it worked.