When your replacement computer is in place and you are ready to remove the old computer from the premises, don't simply hand it off to someone for the purpose of getting it off your hands. Take a few minutes to prepare the system, something that will provide a level of security for both you and its new owner.
Clean Your Hard Drive
For your protection, it is best to delete the entire contents of the hard drive. Use tools such as Darik's Boot n' Nuke to safely remove all the contents. This is safer than formatting your hard drive.
Re-Install An Operating System
You will need to re-install the operating system in order to make the computer functional. If your computer was purchased with recovery software, run the application to re-install the operating system. But make sure it is not an outdated operating system, as support for such are very limited. For example, if the computer is old enough that recovering the operating system means installing Windows 95, 98 or Me, it is probably a good idea to bypass this step altogether, as their support is non-existent and there is extremely limited software available for these operating systems. If the computer originally came with Windows 2000 or Windows XP, it is best to return the computer to its original state.
An Alternative Operating System
In the event the computer did have a Windows 95, 98 or Me version installed, it would be best to seek an alternative operating system, such a a Linux-based OS. The benefits are two-fold, the older hardware runs more efficiently on a Linux-based operating system due to their minimalist design and requirements; and the product is free, with ongoing support that is free as well.
There are many flavors of Linux available and most can be found at Distro Watch. One very popular and highly supported version is Ubuntu. If the computer you are recycling/reusing is really limited in hardware resources, try a lighter version, Xubuntu. Don't need a lot of bells and whistles on it? Try Puppy Dingo. Your options are vast, and the result? A perfectly, fully functional computer for someone in need.