exchange services stops

hi guys,
i have an exchange server which is installed on an IBM PIII 1.4 ghz processor with a memory of 512 mb. the clients stops getting mail service{pop3 and smtp error} at about an interval of 4hrs. when i check the exchabge services (imap4, pop3, smtp, exchange store ) i notice that they’ve been stopped.ive not been able to trace what stops them. everything get working when i restart, but for how long will i continue to do this.