I've heard that your hard drive retains an indeleble record of every single file you've ever had on your PC. The police, for example, are alleged to have the technical expertise to recover any single file that has been deleted.
Personally I use McAfee shredder but I'm not entirely sure if even this is 100% safe.
What I would like is a program that completely wipes all previously deleted files without harming any undeleted files.
I don't want to have to reformat my hard drive as I would lose loads of data and it would be a pig to reinstall.