It can't be done. There's no secure way to donate a computer short of removing the hard drive. The next best thing would be to use a secure formatting system to wipe the drive clean with a program like DBAN, and after that, there's just no way to ensure any reasonable degree of security.
I would like to donate our old computer but before doing so I want to delete all of my documents and any personal information on the computer. What is the safest and quickest way to accomplish this while keeping the existing software in tact? If this is not possible, how does one securely wipe out the entire hard drive?
A charitable computer user!