This happens when you have a mismatch between the Office version on the target, and the Office version at the specified UNC path for that version.
For example,if you specify a UNC for "Professional" and it fails, you may need to enter the UNC spec in "Professional Enterprise" instead, or in "Professional SE" (special edition). Picking the wrong "Professional" version, for example, may result in a dialogue box with the message:
"The feature you are trying to use is on a network resource that is unavailable", followed by:
"Click OK to try again, or enter an alternate path to a folder containing the installation package PRO.MSI in the box below"
This happens because the Microsoft Installer (MSI) you're pointing at with the UNC isn't correct for the installed version of the product, as indicated by registry data on the target. To see what version of Office is on the target, and from where it was originally installed, you can examine this registry key where (HKLM = HKEY_LOCAL_MACHINE):
This key is where the Microsoft Installer records products it installed, so you'll see a variety of entries (long strings) under "Products" as shown below in yellow. On the right you can see my machine actually has Office 2000 Premium on it. Clicking the "SourceList" key will show you the original installation path.
Knowing where this key is will help you fix "Office" deployment problems!