File transfers (USB Drive/Networked Drives) taking so long

I'm moving 204MB (111 items) from a networked (external) drive attached to my
XP machine, over the wireless network to my Vista Machine, and it's taking
over 6 minutes. My wireless connection allows at least 500Kb/sec transfer
rates, so why is it taking so long?

Also, yesterday I was transfering 600MB of photos from my Vista machine to a
USB drive attached directly to it. The transfer took over 12 minutes. I can't
imagine why this should take so long.

I'm using Vista Business. My XP machine is completing these transfers in
less than half the time.

What's the story?