First of all, the performance degradation might be the result of running TCP/IP and NetBEUI simultaneously, as you're effectively doubling all traffic that goes out across the wire. Having said that, there are a number of tuning options for TCP/IP under Windows 2000. Refer to this TechNet white paper, which goes into it in exhaustive detail. A few snippets include changing the TCP window size and scaling parameters options. The default settings are suitable for small 10 Mbps LANs or Internet access where the smallest bottleneck is no more than 2 Mbps. However, for larger throughput: 10 or 100 Mbps, the default parameters might be too conservative and throughput may suffer.
The registry parameters to change the TCP window size and to enable the TCP scaling windows and timestamps options (RFC 1323) are as follows. (All usual disclaimers regarding editing the registry and changing system settings apply.)
These apply only to Windows 2000, as far as I can tell. There are analogous parameters for the window size for Windows NT/98/95, but the syntax might differ.
This was first published in January 2003