To add a little fuel to the fire, here is another option.

It is great that you want to donate the old computer and that would be of little use without operating system and hard drive. So, this is another possibility.

Buy a new HD. One that is larger than the old drive in the computer. If the old drive is, say, 40 gig, buy a 80 gig drive. Put that into an EXTERNAL Drive enclosure and copy all of the contents of the old INTERNAL drive onto the new EXTERNAL drive. Now you have all of your files, Documents, Address Books, Database, Preferences, Settings, Software... all on the new drive to have access to. With everything that you would need from the old drive safely backed up, REFORMAT the old drive so that everything is gone. Then, I am not familiar with this but will defer to the other posts and experience, reinstall all the software on the system disks that came with your old computer.

A note here. I remember hearing, I think it was on Kim Komando (SP?) where someone did a survey. They went to garage sales, flea markets... and other places and picked up a bunch of old second hand computers. The government, I believe, also did something similar. They then took all the drives out of the computers and anaylized them to see what was still on them. To their horror and surprise some had valuable banking and tax information, Credit Card info legal files, secure intellegence information ... on them. Even some of the drives that had been reformatted, I seem to recall had some information that was recoverable. Most of the drives may have been ERASED but, it is my understanding that the information is still there but is tagged, so to speak, "You can erase this if you need the space." That is a message to the OS that the space is available "IF NEEDED". If not necessary that space never gets erased and erased info can be retreived. I have a recallection, maybe I am wrong, but simply reformatting just changes the directories, allocation tables(?) and other information of where and how info is stored. It may not physically erase the data. One way that was suggested that would help is to "DEFRAGMENT" the drive as well. After you have

1.) backed up the old drive
2.) Reformat the old drive
3.) Reinstall the setup, Operating System,(Windows) and other disks from the factory.
4.) Defragment the drive.

By defragmenting the drive you are moving all the new information toward the front of the drive and organizing it all together for a few benefits. First it helps the computer operate more efficiently. Second by moving things around the computer will physically overwrite the sectors etc. where old information might have been stored, as it reorganizes the new information. More possibility that old sensitive data will be zapped from the drive.

There may be software out there as well, check Symantec, that might also clean up a drive and help you insure that your valuable stuff is gone.

Lastly, with the new EXTERNAL drive you will have a valuable item that you can use to store info like Music and Photos... that can often clog up a drive quick. Also by setting your preferences in programs to use the external drive as the location to store your information, most of your valuable data will be already isolated from your system to make a future repeat of this situation a little easier. I have an 80gig Barracuda drive in a USB/Firewire enclosure hooked up to my computer. Being into Video and Photography, (last Saturday I photographed a Hockey Event. 7 games 722 photos taken), I keep all my photos on the external drive. That way, I can keep all this stuff off of my primary drive. I keep a library of all my work and frequently backup everything onto CDs as well. Keeps things a little safer. Also, being external, I can unplug it and take it from computer to computer wherever I go. I have a couple other similar Enclosures with a couple 120gig IBM Deskstar drives for Video Work. The other two HDs are hooked to a PowerMAC G4 that I use for Video Editing. The point is that the extra storage can be helpful for keeping portable archives. Whether video files, Music Files, Chip Collections, Photos, Databases.... After you have gone through all the old information, files, data, software, and moving the stuff you need to your new computer you can then zap all the old stuff off and still have the usable HD space for other stuff.