I just read this over at PC World's blog. I first read about it a week ago but was waiting on a follow-up and here it is.
A major Microsoft techie, today acknowledged a bug with Vista's network programming that can noticeably slow down file transfers on a local network if you play music at the same time.
No word yet on when we might expect a bug fix, so if you're copying a lot of data between Vista PCs at work or at home, you might want to turn off your tunes for the duration.
Go over to PC World and read the rest.
UPDATE: Read How To Fix the Vista Network Speed Issue While Playing Sound