Server access from Win2k Client

What are the possible causes of “slow” writes to mapped network drives on a Win2k workstation using client32? Using a benchmark program indicates that read access speed is acceptable but writes to the server are slow (100-200 kbs when they should be 9 – 10 mbs). Using the same client on a WinNT station connected to the same switch port does not exibit this problem. Problem starts when the transfer size is larger than 4Kb. My possible suspects are; OS – Win2k, NIC Driver – 3C905C-TX Win2k driver, Client or registry setting. Anyone know the cause of this?