Considering that i refused to upgrade to XP from 98 until i found out no shops would sell machines really compatable with 98 so i reluctantly bought my XP machine.
Then did everything i could to hold off the upgrade to SP2, even for a while into the April 2005 when Microsoft made the upgrades compulsary if you wanted to have automatic updates. Only then in May 05 did i upgrade to Sp2.
So based on this probably 2008 i will upgrade. Unless M/S makes it manditory.
But the leap between XP and Vista isnt so great. Vista is mainly a pointless unneeded GUI upgrade. Nothing really new is going to be in it (except according to what i have read more restrictive DRM.)
I am happy with XP and would like to stay with XP for as long as I possibly can.
Or change to linux.