Hi, Mark! It's called the Places Bar, and here's a link to Microsoft that tells you how to customize it:
I did not preview the information to see if it'll do network drives, but I'm fairly confident it'll let you.
When yu click File Open in Excel or Word (I'm using 2000 and 2003) the dialog box opens at the current directory and lists the files and directories. On the left of that dialog box is a shortcut bar, with buttons for 'History', 'Desktop' 'My Docs' etc etc.
It would be nice to be able to customise this to put some shortcuts to some Network drives - but I cannot find any way of doing it. Does anyone know how?