MS Word can make reasonable posters but a decent desktop publishing program is far more flexible. If you've got Microsoft Publisher you could use that but this software is free and easy to use:
http://www.serif.com/...-publishing-software/
Posters are something that it's particularly useful for:
http://www.serif.com/...ing-software/posters/
(Simply use the templates).
Keep your posters simple, using the biggest font size possible. Don't use more than two fonts throughout the whole design. (Use an italicised form of one of those fonts, rather than a new font, if you want something to look a bit different). Remember that people are far more interested in knowing WHAT the event is (and WHEN & WHERE it will take place) than in who is organising it. So the words 'Coffee Morning' should be much bigger than 'Blogsbury District Flower Club'.
Just a practical point, with regard to displaying your posters:
Remember that the ink from an inkjet printer is water soluble. If your posters are to be displayed out in the open, or in windows where condensation can occur, they'll quickly become nothing more than an illegible mess. If you own a laminator then that provides an obvious solution to the problem. Otherwise you'll need to buy a cheap pack of plastic A4 'floppies'. When pinning your posters to noticeboards remember use the floppy upside down, with the opening at the bottom.
Chris