Just to stress the important points about using the Alt codes from the link provided (twice!) above:
(i) the Number Lock on your keyboard must be turned on ;
(ii) you have to use the numeric keypad - it won't work if you use the numbered keys above the letters ;
(iii) if you're using a laptop without a numeric keypad you have to use the Fn key (at the same time as the Alt key) and the numbers which are in the same colour as 'Fn' appears ;
(iv) some Alt codes which you find listed on the web don't always work on particular computers - some trial and error might be called for.
An alternative way to access unusual characters (apart from visiting the pub which I frequent, which is always full of unusual characters!) is to use the Windows Character Map. You can find it from Start > All Programs > Accessories > System Tools.