Windows Vista network mapped drives and shared folders can no longer be seen

My client has a small business computer configuration running Windows Vista Business with Service Pack 1 on three computers, a Dell XPS420 desktop computer and a Dell Vostro desktop computer.  Both computers are networked via a LinkSys router.  The XPS420 is the primary location for the business data files.  The proprietor and her assistant each have a user account on each computer.  The business files on the XPS420 are in a folder that is designated as a shared resource.  Additionally, that folder is mapped as a network drive on the assistants Vostro desktop computer. Each computer is running Norton 360 software.  A printer that is attached to the Vostro computer is shared and is accessed via the network by the XPS 420.

The shared business files have been fully accessible from the Vostro computer since it was installed several months ago and since Service Pack 1 was installed on March 31, 2008.  According to my client, nothing was recently changed on either computer, but all of a sudden the assistant cannot access the shared business data files from her computer.  When I look at the My Computer list on the Vostro, I see a red x on the networked drive.

I tried turning off all firewalls and rebooting both computers, all with no effect  still cannot access the shared business data files on the XPS420.  It should be noted that the XPS420 is still able to access the shared printer on the Vostro computer.

So, what could have happened to squelch shared folder access in an environment where supposedly nothing has changed?  Thanks in advance for your suggestions!