If you are experiencing sluggish network performance after you apply Windows NT 4.0 Service Pack 4, 5, 6, or 6a, then you should check out the following link:

http://support.microsoft.com/default.aspx?scid=kb;EN-US;q249799

Microsoft indicates that the cause of this slowdown is due to a change that was made to the Rdr.sys file in Windows NT 4.0 Service Pack 4 to fix an issue with a handle being invalidated when a dismount occurs. This may cause additional network traffic, which may degrade performance.