Perform a full format of an external drive to wipe away any deleted files. To do so, connect the drive to your computer, right-click it in Windows Explorer or File Explorer, and select Format. Be sure to uncheck the Quick Format box to perform a full format-- a quick format won't fully erase the deleted files from your drive. Repeat this process for each drive you want to wipe.
On Windows XP, data could be recovered from a drive even after a full format. Starting with Windows Vista, Microsoft says a full format will overwrite your drive's data. There's no way to perform a full format from Windows 7's installer, so that's why you have to use a tool like DBAN when reinstalling Windows instead of using the normal Format option.
You can also use other dedicated drive-wiping tools. For example, CCleaner includes a Drive Wiper tool under Tools > Drive Wiper.
Wipe free space
If you've already reinstalled Windows and don't want to wipe your drive and reinstall Windows again, you can try using a tool that wipes a drive's free space, which should obliterate any leftover data left lurking in the shadows. For example, CCleaner's Drive Wiper tool can wipe only the free space on a drive if you'd like.
Just wiping a drive's free space isn't an ideal solution, however. If you have any sensitive files that haven't yet been deleted, CCleaner won't touch them. A full drive wipe is more fool-proof because it ensures everything on your drive is wiped away before you set up a clean system from scratch.
Check your work: Try to recover deleted files yourself
Use a file-recovery program like Recuva, created by the same people who make the popular CCleaner utility, to test whether you can recover any deleted files from a drive. Recuva scans your internal or external drives for deleted files, displays information about them, and allows you to recover them. Be sure to perform a "Deep Scan" when prompted--it's slower, but will find more bits of deleted files. If you wiped the drives properly, Recuva should find no files you can recover.
Recuva performs the same sort of trick an attacker would use to recover your data. Of course, some attackers--particularly criminal organizations that target businesses--may use more advanced disk forensics tools to get at that sensitive business data.
Use encryption to protect all your files