Why is initial access slow on network?

I have recently added a Windows 2000 w/SP1 system to an NT4.0 w/SP6A network. When opening a file by double clicking in explorer the file takes an unusually long time to open (30 seconds or more for a 20K file). When opening a similarly sized file through the application the file opens almost immediately (as it should). It does not matter what the application is as the problems exist using MS Office, TIFFs or Adobe Pagemaker files.

The Server and the W2K system are using TCPIP. All other machines on the LAN are NT4 or W95 using TCPIP and do not experience the same problem.

What could be causing the problem?