There's a lot of text-to-speech programs out there. I'm not sure if any of them will read right from Word, but I imagine some would. If nothing else you can copy-paste the text into them. I think the one I used was called ReadPlease, but there are a few and some of them are freeware. Download.com has a few I'm sure.
They're fun for pranks but with the free ones it takes a bit of fiddling to get them to sound as good as possible, and they'll always be noticeably synthesized. Some of the non-free ones apparently have voice packs that sound pretty natural, but for my phone purposes I wanted that Stephen Hawking vibe.