Wipe your drive and reinstall Windows 7
Windows 7 doesn't have this wiping feature built-in. If you just reinstall Windows 7 on your PC using a Windows 7 installer disc or your PC's recovery feature, your drive won't be wiped. Deleted files could theoretically be recovered from your drive.
To avoid this, you'll want to use a disk-wiping tool like Darik's Boot and Nuke (DBAN) before reinstalling Windows. This tool wipes your computer's hard drive by overwriting it with junk data. If you're disposing of the PC or internal drive, you're done—you can leave the PC in this state. If you're passing along the PC to someone and want to give them a working copy of Windows, you can then reinstall Windows on the PC and pass it along.
For a full rundown of DBAN and other secure erasure tools, check out PCWorld's guide to securely erasing your hard drive. Be careful when using tools like DBAN! They will overwrite an entire drive, including any recovery partitions and other data you might want to keep. Back up any data you want to keep before wiping your drive.
Clean external drives
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.