Slow Responses in Exel

File Sharing on W2K Server.
We are having a terrible time sharing Excel files on a W2K server. Clients are running WinXP Pro and accessing files from the server starts out fairly fast. As the day progresses, the time to open and close a shared file gets slower and slower until it becomes almost impossible to open or save the file. If we reboot the server then everything seems ok for the rest of the day, until sometime the next day and it starts over again. I have checked all sharing parameters at the server level and file level, but nothing seems to help.

What am I missing? Sharing files has almost become an impossible task on our server.