Only someone unfamiliar with corporate IT would even ask this question. A fundamental tenet of corporate IT is "not fixing what ain't broke". If your Win2K (or even Win95) systems are meeting your corporate needs, the cost/benefit analysis quickly says "don't upgrade". (Fry's point-of-sale and inventory systems run on MS-DOS today....)
Vista basically needed more cpu/memory/disk/graphics than a typical 3 year old PC provided at the end of 2006. That meant that the first step in a Vista migration would be to upgrade hardware on hundreds/thousands/10's of thousands/100's of thousands of desktop.
However, today Windows 7 runs fine on 3 year old PCs. (Note that today's 3 year old PCs were new when Vista was released.)
OMG, somewhere actually know the intricacies of corporate IT.