Vista SP1 vs. XP SP2 - Part Deux
 

One of the main criticisms of my initial benchmark tests was that I overlooked the fact that, under the hood, Windows Vista SP1 works differently to XP SP2 in the file copy department. The difference is down to caching and how Vista carried out uncached I/O while XP caches the files. On top of that is the further complication that the file transfer progress bars are coded to work differently. Vista’s file copy dialog box goes away when the cache is committed, while under XP the copy dialog goes away while the committal is still pending. In other words XP is coded to appear fast.

http://blogs.zdnet.com/hardware/?p=1338