I've seen the same problem on our network. I've heard Samba 3 fixes all this, but I haven't had a chance to try it out yet.
I did notice that machines we upgraded to XP from 2k didn't suffer from the speed problem, though-hmmm...
(I don't run Win at home at all any more, and I'm a little over-cautious about changing our production systems at work...
)
BTW: I've been told there's a registry entry that persists for QOS, even if you delete it off the system. I'm not sure what it is, but I could probably dig it up...