Slow UNC or mapped copying even using only local resources?

[Brenda]

We have some kind of I/O issue on our Server 2003 machine.
At first I thought it might be a network problem but, based
on my experiment below which involves no network traffic
at all, I think it's something else.

Here's the experiment:

1. Create two folders on the desktop called 'A' and 'B'

2. Turn on sharing on folder 'B'

3. Place a 200 MB test file in folder 'A'

4. Copy file from 'A' to 'B' -- takes just a few seconds.

5. Click Start/Run and run \\server\b

6. Copy file from 'A' to \\server\b -- takes ten minutes!

7. Map drive Z to \\server\b

8. Copy file from 'A' to Z: -- also takes ten minutes!

Why the slowness with mapped drive or UNC even on the same
local machine not traversing the network? Is the NIC still
involved somehow even with only local resources involved?

So far, I've found the references to SMB and TCP ACK issues
and even tried the suggestions with no effect on this performance
problem.

Can anyone help?

Thanks.