Reformatting isn't enough to truly delete data from your hard drive, you need to use a disk-wiping utility. Luckily, though, there are many recommended ones that are free.
From the blog [Geeks Are Sexy] comes recommendations for 3 programs you can run that fill your hard drive with random information, no matter the type of computer you have. This is important because even if you reformat your hard drive, there are programs to pull that information off the hard drive.
These programs work because they fill your hard drive with "white information," effectively writing over any information that remains on your hard drive.
Once it's been written over, it's impossible to get off. Then you can feel safe to recycle your old computer by donating it.