The MPW isn’t required with winXP that is why it doesn’t appear, the set-up for email accounts isn’t required in winXP.

Set your accounts as they should be without the changes made in your previous win98 system and that should do.

Are you experiencing any problem sending or receiving email, errors, etc. ?
If not and the avast mail icon is present when downloading email and the avast icon rotates then it is working as it should.