Where's file open notification in excel 2000?

I have a Windows 2003 server R2 Sp1 and XP SP2 workstations with Office 2000. Occasionally 2 users are able to access the same excel 2000 spreadsheet at the same time without receiving a file open notice until the second user tries to save the file. Then they receive a message stating that the file is in use and that they can do a save as. Usually if a user tries to access a file that is already in use, they receive a file in use popup and they are given the choice to open read only, notify or close. This issue does not happen all the time and is not specific to one particular file. And the file was never setup for sharing. I've checked my security and permissions for the shares and I don't see a solution. I've suspected file corruption and copied the data into a new file and replaced the old file thinking that would solve the problem and no luck. Is there something I'm missing?