DOS (text format before Windows) offered a range of character sets called "codepages". They all shared the same codes for the standard English alphabet (a, for example is always code 97) and common punctuation symbols, but included varying symbols for box-drawing, foreign language accents, etc.
If you process texts in German, Spanish, Russian, Greek, Polish, etc. you may need to find out which codepage was used when the texts were originally typed.
For example, the character ã is coded one way in codepage 850 (Multilingual) but differently in codepage 860 (Portuguese). It is simply not available at all in codepage 437 (the default codepage in the UK and USA). To alter or examine codepages, see a DOS manual or check the topic out on the web.
When it loads up, Oxford WordSmith Tools detects the current DOS code-page, so the codepage is only likely to need altering if you are using texts produced when another codepage was in use.