Diskless is the way to go. CHeapper to run because you dont need an OS or hard drive for each machine. Which woks out cheaper to run as well because the HD uses a lot of energy.
Here's what you need
And Doubletop over at TPR has a guide on using it.
LTSP Diskless Farm Setup
Works very nice. He has about 20 nodes (or did have) on it crunching away. That can run BOINC and Folding@home. Not sure how to setup other mind.
Another guide here. (Folding only mind)
If your using XP already and dont want another PC to be the server, you can use windows network boot option. Again folding only but I'm sure you can find a way to do others.
Magellen - Intel E5300 @ 3.20GHz ~ Gigabyte GA-73PVM-S2H ~ 2Gb RAM ~ 320GB + 250GB ~ ATI 3850HD ~ Coolermaster Elite