My sister reports on her new PC (WinXP SP2) that she can’t open and read her emails from a well-known source the Quebec Medical Association. Are they sending infected emails? Is this linked to Avast? 
Unless you give us more information on, the operating system, the exact error messages, the mail client then answers will be useless guesses.
Add to that.
What is your firewall ?
Does it allow ashWebSv.exe internet access ?
- If it does delete the entries for them, reconnect to the internet and try to collect your email, this will force the firewall to ask permission again.
Is it webmail viewed through a browser or pop3 downloaded through an email program ?
She uses Outlook 2003, has no firewall but uses WI-FI and WIN XP SP2 and AVAST HOME EDITION
Is she using the Outlook/Exchange plugin (which is recommended for Outlook users) or the Internet Mail provider?
By the way what is the error message that she is seeeing?
Make sure the avast plugin is not disabled into MS Outlook:
Outlook 2003 > Help > About > Disabled items.