You can also access characters with accents, etc via the Character Map. (Start > All Programs > Accessories > System Tools > Character Map).
However I usually use Alt codes. You need to have a keyboard that's got a numeric keypad on it. (Alt codes don't work with the numbered keys above the letters although, if you're using a laptop without a numeric keypad, you can use the coloured numbers with the Fn key instead). You also need to ensure that the keyboard's number lock is on (as it usually is by default with most computers). Then you hold down the Alt key while typing the relevant number from this list:
http://www.tedmontgomery.com/tutorial/altchrc.html
When you release the Alt key the required character will appear.
For example, holding down the Aly key and typing 164 produces ñ. Typing 168 instead produces ¿