My recommendation would be to create a new user (administrators group users) and log on as that user. Make sure that you have backed up any files (that you may want to keep on external media) from the accounts that you want to remove. Open the Control Panel and select the User Accounts applet. Choose the "Manage another account" link. Select the accounts that you no longer wish to leave on that computer that you are giving away and Choose the "Delete the account" link, and then the "Delete Files" button.
After clearing all the unwanted accounts and their associated files use a program like Ccleaner to wipe the drive's free space. Launch Ccleaner and select "Tools" then "Drive Wiper" from the left column, make sure "Free Space Only" is selected, check the box next to the appropriate drive (usually Local Disk C) and hit it with three passes (more if you're seriously paranoid).
Welcome to the forums and I hope this helps
Regards
Randy