For drivers and programs, Win2k vs WinXP is going to be like Win98 vs WinME. Drivers are software should be exactly the same.
You lose the new look...you don't get system restore...XP apperntly has better Digital Camera support...a few things like that....nothing major though. By and large Win2k = WinXP for drivers, and software/hardware support.
Windows 95, 98 and ME = Windows 4.x. Most companies still support windows 95 just fine in their WinME programs.
Windows 2000 and XP = Windows NT5, I'm sure all companies will support both.
<< wonder if after XP comes out will Microsoft still support it with service packs and such >>
I'm not sure if I should tell you this, but I know a guy that is on the Windows 2000 project team for my company (we are at NT4 still), and he tells me, this:
Windows NT4 (and I believe Win9x/ME as well) support ends as of 2003.
Windows 2000 supports ends as of 2004.
So I assume that means Microsoft will continue to produce patches and service packs. And if they are making them for bigger corps, I'm sure you can get your hands on them for the average consumer.
I think it's safe to say Win2000 has several years of life left in it.