Just reinstalled Windows XP Home SP2 on a Dell Dimension 2400. I'm now it's third owner again. It started off with a nice lady in a local town who wanted a laptop instead of a desktop. After a few months, she wasn't using the 2400 anymore and wanted it to have a new home. With it came a 15" LCD, and I gave that to another lady who was using a 15 or 17 inch CRT that only showed in BLUE! Now, she gets Vista in full color. It's going to replace the PIII-933 that I had on my patio for streaming audio out there.
This Dell is five years old, still in good shape and needs a job. So, a great thing to do that need minimal processor power is make your old computer into an audio playing station. You can rip all your CDs to the HD and use jukebox software, use Winamp and connect to radio stations around the world or right next door to play music, and then get a pair of outdoor rated speakers. I use a Rubbermaid cabinet for the computer, stereo and speakers, and have a switch there for other computers if I have friends over.
And with a video card and processor that support it, you could also use the same computer for streaming video or playing DVDs with an s-video or HDMI cable, or use a larger monitor.