Hey there JR1955.
I too am in the habit of taking in the orphaned PC refuse of those around me, fixing them up, and then releasing them back into the wild.
I my experience, it's never enough to simply delete the cookies and check for malware. The whole system needs to be wiped. (there's a charity accross the bay from me that does this too, they actually have a degausser which is basically a powerful magnet that instantly wipes hard drives).
Of course after you wiped the drive, then what? I'm sure that just like me, you don't happen to have dozens of Windows licenses just lying about waiting to be installed on these machines. And Microsoft has taken people (and charities) to court for installing 'pirated' copies of Windows 95 on old computers (and this was in, like, 2005!).
The obvious solution is to install some flavor of Linux. Personally, I prefer to put Ubuntu on them. Some really old computers can't handle the latest and greatest version (7.10) but they just thrive under older versions (6.10 is still supported and is quite good.)
In my dealings in the past where I donated such a computer to worthy causes/people, I have never received a negative reaction to Ubuntu. Indeed, most of the people to whom these computers have gone have never been able to afford a computer and so don't have to battle against the "Windows to Linux" learning curve that others do.
All that aside, there is one other very good reason to wipe the hard drive (thoroughly): privacy.
I'm sure you know that a file is never *really* deleted, it just gets a bit harder to find. This is true even if the hard drive has been reformatted. In order to protect the people from whom you get your old computers, you need to have a solid data destruction policy in place and in practice.
I use Darik's Boot and Nuke
, an excellent program that 'nukes' hard drive data. It can be downloaded from above or incorperated into the Ultimate Boot CD for Windows
which includes just about every tool you'd ever need to save-or destroy-data on a hard drive and should be in every IT person's toolkit.