there is not much performance difference between W2k and XP.
Apart from that, XP Pro gives some more administrative options that allow a higher security level. Additionally, there are some softwares that don't work on W2k
And not to forget: MS will stop delivering security updates for W2k next year. Extended support for XP ends 2014, if I remember correctly.
So, instead of installing W2k, I'd see if I could tweak the system somewhat. Disabling unnecessary services, avoiding installation of bloatware does help.