No, Vista/Seven have lots of heavy changes in kernel, media handling, etc.
For example, having MPC-HC decoding HD video using the GPU instead of the CPU works fine in Vista, but I could not get it working on XP.

They made mistakes in Vista, some things are heavier for nothing, but there are also many changes. Like there were in XP SP2 compared to original XP, or in XP compared to 2000. You do not notice such changes when you upgrade, but when later you have to make things on an older PC which still runs the previous Windows version, then you notice that there were new/improved things in the new Windows