It can be done using Performance monitor. Simply set up a counter for logical disk free space. Then set an alert, and under the action option you can either have it send a network message or to send an e-mail do the following. If you want an e-mail than under the action option have it execute a program. Have it to execute microsoft outlook express or some other POP3 mailer that you will use ONLY for this purpose. In the mailer app. create an outgoing message that warns you of low disk space. Then set the send recieve option to less than a minute. Close the mail app. you are set. Thats one quick and dirty way. The other is to use SNMP but its a mission.
Honestly, third party does it best.