XP was an OK OS in 2001.
It's still ok. 71% marketshare last i checked.
I have XP , Vista, and 7, I MUCH prefer XP with respect to customizing possibilities, explorer file navaigating and start menu design.
Yeah, 7 is a a brand spanking new OS, but that doesn't in any way make it preferable. Design wise it's pretty much the same as Vista with a few new bits added to boost sales imo.
Security wise, 7 is better and if you have a nice fast machine may run a bit faster.
If you are happy with XP, I'd stick with it.