You'll find this should wipe clean all the unused space on your system.
Be careful you only use the option to wipe clean the empty spaces. It's shareware, but from a reputable source, and if you just use it the once, it won't cost.
The remnants will gradually get overwritten by new files.
A defrag will shuffle all the data around, and also quite likely overwrite much empty space. And is a useful thing to do from time to time anyway.