Questions, how much did you pay for these machines?
What spec were they, when you bought them, what spec are they now?
How much are you thinking of selling these machines for?
Did you purchase a separate license key for each pc your selling?
If the answer to the last question is yes, then I think you could be onto a major loss. As I cant see any value in a win 2000 spec machine, (obviously dependant on spec), together with upwards of $100 for a XP license, and the time taken to install windows etc.
I would be very interested to hear your answers to the above please.