do others have physical access to the comp? if your not logging out correctly and someone has physical access to your comp then of course they can read your mail and anything else you've done..if you are logging out of your accounts correctly then you may be infected with a trojan/keylogger or similar program run netstat and see what open connections you have , in that case. check for any unknown or suspicious connections or listening ports. As for running a key logger on your own comp, not advisable.