A little of both.
Mostly, it is a Windows 2000 refinement. It is actually listed as Windows NT 5.2 in some places in the OS, as Windows 2000 was defined as NT 5.0.
There are a lot of differences between 2000 and 2003, but they are mostly just small improvements. IIS 6, however, is totally revamped and all new.
SeanFlynn