Chances are, your system runs software that keeps logs of every site you browse on the Web, every document you work on in any application, and every user name and password you type. What program is doing all this spying on you? It's called Windows.
Because of the sensitive information your Windows logs invariably contain, it's a good idea to purge them regularly. But if you're giving your PC away, it's essential. Of course, Windows isn't the only software that can betray you if you donate your hard drive--old financial documents are a dream come true for identity thieves. And even if you drag those old tax returns to Windows' Recycle Bin, plenty of utilities can retrieve them. The only way to ensure that you've taken out the trash for good is to use hard-drive-cleaning software.
After trying several programs, I've settled on a small tool kit that can help clean up a sloppy computer at a moment's notice. Which utility I use depends on what, exactly, I need to accomplish. Case in point: If you're donating your PC to a school or charity that needs the Windows installation to remain intact, you can't simply overwrite the hard drive. That's where Webroot Window Washer 6 ($30) is extremely useful. This handy utility can delete sensitive data from history lists, file caches, and logs created by more than 450 different third-party applications, without touching your Windows installation.
Of course, you'll still want to securely delete all the non-Windows files on your hard drive, especially any documents you created. Eraser 5.3 (free) has a bare-bones user interface, but it's the most effective free file-deleter I've found. It works by overwriting files with other data multiple times (four options let you choose how many overwrites take place) to ensure that the data can't be recovered. You simply drag any files or folders you want to delete into Eraser's window, and then click Task, Run.
If your system's next owner doesn't need your Windows installation, your best bet is Darik's Boot and Nuke (free). Boot and Nuke does exactly what its name suggests: Once you've installed the appropriate files on a bootable floppy or copied them to a CD, just boot the computer with the DBAN disc to wipe the hard drive completely clean. Its advantages are speed and ease of use, but DBAN can't delete individual files, so it's suitable only when you want to get rid of all the data on an old hard drive.
It's good to find a new home for your old PC. Just make sure the machine doesn't tell any of your secrets once it gets there.