Just to add to the previous note -- if you are using windows and also SMTP authentication it is possible to get this to work by using the PEAR Mail package:
http://pear.php.net/package/Mail
I do not believe the built in Mail function provides this.
The sendmail binary requirement only applies to Unix-like platforms (e.g. Linux). In Windows you need to specify a mail server that accepts mail through a network connection. Find these lines in your php.ini file:
[mail function]
; For Win32 only.
SMTP = localhost
smtp_port = 25
; For Win32 only.
;sendmail_from = me@example.com