Before someone gives you the stock, stupid, answer of installing a French
keyboard layout, if you only need to type in Word, there are many intuitive
shortcuts built in to Word.
e.g. to type á, you type Ctrl+' then a.
Look up "keyboard shortcuts for international characters" in Word's help for
more.
If you're writing text in French (rather than just correctly spelling
loan-words such as café and résumé), then you may want to manually change the
langauge of the text you write (tools > language > set language) so the spell
checker knows what it is, although it should auto-detect.
If you need to type in other applications you will need to condsider another
option, such as installing another keybaord layout. Installing a French one
is not going to be a good idea unless you are a touch-typing genius with a
photographic memory, otherwise you'll never remember where all the special
characters are (for that matter you'll have a job finding the "a"). But you
can install either the US International or United Kingdom Extended layouts
which both let you type most characters you would need in French. you can
have more than one layout installed and switch with left alt+shift
Do this in Control Panel > Regional and Language Options > Languages >
Details > Add
With USI, you type e.g. ' then a for á, " then e for ë etc
With UKE, you type AltGr (right alt) and ' then a for á, AltGr+2, e for ë etc
The UK Extended the better one, as with the US International you will find
yourself repeatedly typing things like ä quote" instead of "a quote".
You can look up the keyboard layouts (and see why you don't want the French
one) here:
http://www.microsoft.com/globaldev/reference/keyboards.aspx