Typing an email doesn't count as activity, there is no data sent over the connection when typing. If you are getting a message FROM your Windows XP OS, then look at your dial-up settings for a disconnect time-out and either disable it or increase the time period. There may also be a separate setting on the modem properties/settings. Sometimes Windows just goes wonky and triggers a time-out disconnect when the settings ARE disabled. If the setting(s) already are disabled, toggle them ON, then back OFF, that may get it back on track.

here's some steps to lead you to dadoes fix.

Here is a link that might be useful: http://help.expedient.net/dialup/disconnects.shtml#idle