using CHR(), A = chr(65), B = chr(66) and so on.
If you'll be using this conversion in multiple places throughout your DB, you can write a function. But just to test it, in a query that is based on your table, you could put this (sub in your ID field name for CountryID):
Expr1: Chr(65+([CountryID]-1))
and it will convert; do you have fewer than 26 countries? If not, you'll have to write a function that doubles the letters to like "AA", "AB", etc. Hope this is enough to get you thinking.
g