Internet Mail and Instant Messaging providers only? I.e. the Standard Shield provider is stopped? Strange…
Can you please try to check with avast! installed, but stop even the other providers - especially Instant Messaging… could it be the one causing the troubles? Or just a mere presence of avast, with all its modules disabled, would cause the conflicts?
You may also try to stop the avast! services from the WinXP Control Panel temporarily… to see if it has any effect.