Agree with the_gooner that getting a new email address is probaly by far the simplest / cheapest solution to your dad's problem.
He will need to spend a lot more if he goes the broadband route - Tiscali offer a basic package for �6.49 pm but that increases to �12.99 after 3 months. Doesn't sound like your dad would really make much use of a BB deal, I would stay with dial up and just get a new email address. Why not try web based email such as Google Mail, Its free and has built in spam filtering?