"cannot access" error when opening excel doc over network

ok here's the deal...

There is an excel document on a server running microsoft server 2003.
There is a windows machine
There is a mac.

If I am writing to the excel sheet on the windows machine then I can open the excel sheet as read-only on the Mac... this is fine and to be expected of course.

If I am writing to the excel sheet on the Mac and I try to open the excel sheet on the windows machine i cannot even open it as read only... instead I get the error- "Cannot Access: filename.xls"

I only want to be able to read the excel sheet on a windows machine while it is open with write access on the mac.

The docs in question open fine on either machine when they are not in use by the other.

...

This issue reproduces with all flavors of Win Xp Pro and Vista and with OS X 10.4 and 10.5... it reproduces on many different machines, with every excel and/or word doc I've tried... but only Microsoft Office docs.

On Windows we're using Microsoft Office 2007... on Mac we've used Microsoft Office X and Microsoft Office for Mac 2008.

The Microsoft "personal support" service was unable to resolve this issue and they want to elevate to the "pro support" level,... but I'm trying to do some research elsewhere before dropping so much cash on this small inconvenience.

big thanks if you can help!