What do you mean "playing with his pc"?
Do you mean always tweaking it? Messing with configurations and hardware settings? etc.? If that's what you mean, then
definitely go Win2k. Without a doubt.
Do you mean play a lot of games? I'd still say Win2k. But it could go either way.
Or like GZ said, do you play a lot of OLD games? If you do then I'd have to say...why? Why do you play old lame games on a new computer and new OS? Move on. Join us in the now. But if you won't give up Commander Keen then I guess maybe XP might be a better choice.
Personally, every time I have to deal with XP, at friend's houses, in the office place, anywhere...whenever I have to use XP I always come away from it with a new found appreciation for Win2k. I am never gonna like XP. I will hold out on Win2k until the next generation of OS or maybe just forever.