Is it really wise to alienate any of your consumers when you're already losing subscriptions at an alarming rate? This seems like a poor course of action.
Even assuming just 5%(probably realistic), that's a hit of over 400,000 subscribers - literally overnight. I don't think this is a wise thing to do.
If you can run Windows 2000, there's no reason you can't run XP. The hardware requirements are basically the same between the two.
Better yet, buy a new machine. Your game experience will improve greatly coming from a Windows 2000-era computer. I guarantee it.
The problem is that since Windows XP has been discontinued as well you're not able to purchase it from normal retailers. The only retail version of Windows being sold at retail is Windows 7 which won't work well at all if you have Windows 2000 era hardware.