Have you got an old computer gathering dust in the attic? Did you know you can revive it with a modern Linux operating system? It can become a useful router/firewall, server or even desktop computer again. All without buying expensive Windows licences that are no longer even supported. Old limitations on file names, disk size, USB support in early Windows versions can even be overcome. You can put a 21st Century operating system on a 20th Century computer.

[edit] Steps

Consider what you want, a desktop computer, a server, or a router/firewall.
Clean it with compressed air.
Test your PC will turn on safely.
Determine what your PC will boot from (in the BIOS or manual), old ones might not boot from USB, really old ones might not even boot from CD.
If it won't boot from CD, download floppy boot images of Damn Small Linux and Puppy Linux (WakePup for Puppy Linux 1 and 2 series) and once extracted onto a floppy disk, insert them into your old PC.
If you're on dial-up and don't want to download Damn Small Linux (50Mb) or Puppy Linux (100Mb), then buy a CD online.
Download SliTaz, Damn Small Linux or Puppy Linux CDs (on a new PC if you have one) and burn them as images (with Infrarecorder or Nero).
Turn your old PC on if you haven't already and insert the CDs as soon as you can, if successful, you'll be greeted by a DSL or Puppy Linux boot screen for a few seconds (press a key quickly to interrupt the countdown if you like)
Unless you have valuable data on the old PC, consider creating a swap partition (with gParted or Ultimate Boot CD) if you have too little ram (less than 64Mb) to run the distros "live".
Read and consider carefully using cheat codes at boot (also called boot parameters) to run more from CD and use less RAM particularly with Puppy
Choose Xvesa rather than X.org framebuffer if you have display problems
Turn off ACPI or APM if you have problems.
If you like Puppy Linux but 3 series is too slow, consider 2 series (Phoenix or 214R), or even 1 series (MeanPup or 109CE).
If you like Damn Small Linux, also consider Damn Small Linux-Not (with Abiword and Gnumeric) or Feather Linux (also derived from Knoppix).
If you have a serial mouse, test whether it is detected and works. If it doesn't, then additional configuration can be made with boot parameters/cheat codes.
For dial-up modems, consider a hardware dial-up modem for maximum compatibility.
Some gains in performance may be made, once installed to hard disk rather than running live.
Also consider a "frugal installation" for performance gains.
If you spread the word, tell your friends and share Damn Small Linux and Puppy Linux CDs, you can build your own local support group to help revive old PCs.

[edit] Tips

If you're adding applications, choose lightweight ones like SIAG Office rather than OpenOffice.org.
Choose lightweight window managers like JWM, IceWM or Fluxbox rather than GNOME or KDE.
Choose lightweight browsers like Opera or Dillo rather than Konqueror or Flock.
If you want something different and unusual with eye-candy, consider Enlightenment window manager which comes with eLive.
DeLi is good with older computers than Slitaz, Damn Small Linux and Puppy but is not live and is harder to install as it requires manual text based partitioning.
This old pc can be used as a file server, router, and more.