What's the best way to completely remove everything from a computer, without re-installing?

You're not going to be able to clean a computer to any degree that is secure without wiping and re-installing the OS. However it doesn't have to be painful.

1. Download and install to a CD Darik's Boot And Nuke.
2. Get hold of a copy on CD of the version of Windows your friend is using.
3. Download and run Double Driver. Use it to back up your drivers onto a USB stick.
4. Use Magical Jelly Bean KeyFinder to find out what the Windows XP serial is.

Once you've done all that, then you're ready to go:

1. Run Dariks Boot and Nuke to erase the hard-drive. As noted by M.M this could take many hours, so run overnight.
2. Boot the Windows XP CD and go through the installer. Ensure that Windows Updates are turned off (as otherwise you'll find yourself wading through hundreds of them) during the install.
3. Use the CD key you got previously to re-register Windows.
4. Let Windows go online and validate your CD key.
5. Unplug the network cable.
6. Point Windows at the USB stick for any drivers that are missing. No need to download anything.
7. Turn on Windows Update to the recommended settings.
8. Power off the computer.

Job done. If you're feeling benevolent, then you could also:

1. Turn on Windows Update and download all high priority packages
2. Install Microsoft Security Essentials.
3. Use Ninite to pre-install a bunch of useful applications really quickly and unattended (eg. FireFox, VLC, Picasa, Adobe Reader, Flash, SilverLight and CDBurnXP if you have a CD/DVD writer on the computer)

This is what I have done in the past before donating an old computer to a friend or a relative.

1. Uninstall everything that isn't shipped with windows using an uninstall tool that also cleans the registry and deletes leftover files. (I like Revo Uninstaller Free)

2. Delete all bookmarks in IE

3. Use a tool to remove private data from the computer (browser history, cookies, recently opened files ..) Again I recommend Revo, It has a Tracks Cleaner on the Tools tab

4. Delete all files on the desktop and in my documents and anywhere else you know there is data

5. Empty Recycle bin

   If you have more than one user, repeat 2-5 for each user

6. Create a new Admin user, with a blank (no) password log on as that user

7. Delete all other accounts

8. Delete the folders for the old/removed users in C:\Documents and Settings

9. Delete C:\Documents and Settings\All Users\Dokument and all files on any other partitions/hard-drives beside C:

10. Do a full computer search for media files and documents to see if you missed anything interesting and delete those

11. Get a "free space" scrubber and run that to make undeleting files impossible (I use Eraser)

After this the computer should be safe to sell.

couple of options

• just wipe the entire hard drive with a utility like Darik's Boot & Nuke (http://www.dban.org/) which will effectively blank the entire drive. Then allow the new owner to install whatever operating system they'd prefer.
• create a new user, assign it admin rights, and then remove the contents of the folder(s) located under C:\Documents and Settings{all user EXCEPT the new ADMIN} for example, remove C:\Documents and Settings\administrator. This in effect removes all the personalized data for the users. Then you'd have to get some sort of registry cleaner, or as I've done in the past - manually walk through all the registry keys and remove them within REGEDIT. Painfully cumbersome, but effective.

In reality - the safest way to dispose of a system is to WIPE it using a software disc eraser. Any software method can leave fragments of files behind since the file is really NOT removed, that is, anyone with time and a little knowledge can RECOVER deleted data.