If you want to keep the operating system on it and the various programs then download CCleaner (for free)
https://www.piriform.com/ccleaner
Then do a basic "Run Cleaner" (press the Run Cleaner button) first against Windows then against Applications. This will get rid of a lot of the "dross".
Then still within CCleaner, within the Tools section, run Drive Wiper and choose the option "free space".
This will wipe out anything on the free space (empty space) on the disk, but leave all the other files like Windows and programs.
If you want to do a complete wipe of the disk then run DBan (Dariks Boot and Nuke).
http://www.dban.org/
You need to create a boot CD and boot the computer from that and run the program.
However as has been said, this will wipe EVERYTHING off the disk so you will need to reinstall Windows and any programs which could be a pain if you need to locate and install new Windows software.
You could sell them with "empty" disks but it may be that the software on them (Windows and perhaps Office) is worth more than the hardware.