Outlook Express 6.0 began to arrive only with attached msgs

Good evening.
I am still running an old PC with XP and Outlook Express 6.0.
Everyting ok until recently the messages began to arrive not in the screen but in a txt. attach. How to return to receive messages normally, that is, it is able to read the messages not attached, but in the message body.
Thanks in advance. I am using AVAST.