Office programs – slow to open – why?

Here’s the scenario…Office XP suite running on Windows 2000 workstations. Files are stored on a Windows 2000 server. Files opened from the mapped network drive open with significant delays initally. If same file is opened again after closing, it opens immediately. Symptoms seem limited to Office products. Server and workstations are running Trend Micro virus protection…not Norton. Machines are newer and relatively well equipped. Anyone seen this before? Thanks for your help!