I think you can right click the hard drive in My Computer, select properties, then make sure "Compress drive to save disk space" is unticked. It may be ticked and grey if only some files are compressed, in which case click it a couple of times til it's clear again. If you have a large drive or lots of compressed files, the operation could take a while. I'm not sure how "deep" this goes regarding subfolder trees or some compressed/some uncompressed files, so there may also be another way.