Simply deleting files on your computer, or even reformatting the hard drive does NOT erase all data stored on the drive. When you delete a file it isn't really removed from the disk. The file content remains on the disk until another file is written over it. Basically the same thing happens when you re-format a hard drive. Most of the data remains; the space on the drive is just made available to be written over. The Windows delete command only removes bits of information from files so they appear deleted in OS, but those files are easy to retrieve using specialized file recovery software. In order to remove, or shred files permanently from your system, you have to use a program that is capable of rewriting the files with random series of binary data multiple times, thus the term "shredding."
To be really safe, you have to overwrite/erase/wipe everything. Also, the Windows swap file could contain data you wouldn't want to have fall into the wrong hands.
A good, and easy, program that has both free space wiping and file shredding capabilities is File Shredder. It has a small download size, simple interface, and is very easy to use. Minor drawbacks are it lacks scheduling, a help file and the online help is very limited, but normally, you won't need any help.
File Shredder uses something called a DoD erase pattern by default, but it also has four other patterns to choose from, each one stronger than the previous one. With File Shredder you can remove files from your hard drive without fear they could be recovered.
File Shredder was developed by the author to shred company files but he released it free under the GNU license. Recommended.