RAJIV7

I have a 320GB P4M900M2 hard drive. I am using windows vista Home Premium.
When I install Vista hard drive capacity came down to 298GB. Then I have
transferred some files (MP3) from my old computer 20GB. Now my available
space should be 278GB but for some unknown reason every time I use the
computer (listen to music, paly DVD) the hard drive space come down. It came
down from 268GB to 236GB then I have deleted all the files I have transferred
20GB and emptied the recycle bin. But still it shows as 256GB free from
298GB. I have done a search but can't find any big files. 38GB missing from
the hard drive. Why does it do this? Is this a problem with my hard drive?
Please help.