The information in the following MSKB article may help you:

You cannot save a file from your Windows XP-based or Windows 2000-based
computer to a shared folder on a file server
http://support.microsoft.com/?kbid=895900

fryb53 wrote:
In the past I have been able to save a new excel file back to the desktop
server. Now when I try it indicates that the save location is not available.
But if I save the file locally first and then back to the desktop server it
saves ok. Is this an excel setting or some network condition preventing save
of new excel file? Thanks.


--
Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html