Searching File Contents on a networked M/Soft PC via Spotlight

I am new to macs and absolutely love it so far I do have one problem, I have a network with two PC's on it containing Word Documents that I need to be able to search the contents for a particular word or phrase. This not possible with Spotlight, and have been advised by Apple Support to post this here as then hopefully someone will recomend a solution.

Example.

A document will contain the words "Bank Of England", The document is stored on a networked PC, I can see the PC but cannot search through Spotlight for the words in the contents.

Any help would be greatly appreciated