The function you are trying to use works with sendmail, which is a mail server for unix-like systems. In unix/linux/bsd/solaris/uix and so on it is found I believe under /usr/sbin/sendmail.
Unfortunately, in Windows, sendmail does not exist. Therefore, when PHP is looking to find sendmail in Windows, it simply cannot (in your php.ini there are some variables like SMTP, Sendmail_Path etc. As I said before, "sendmail_path" for unix machines =/usr/sbin/sendmail.)
A simple way to go around this, is to use an online SMTP server which you have access on (not gmail or hotmail I'm afraid, has to be some commercial SMTP server I think, one that you pay for), and use the SMTP and port variables to configure that.
If you absolutely need to send email from your windows machine, then you must install an smtp server. There are many good ones out there, I'd recommend Fake Sendmail, which is like the unix sendmail but configured to work under windows.
After you install your mail server, define the path to its executable from your php.ini, as said before under "Sendmail_Path". Also, you will have to do some configuring from the mail server so you enable it to send emails.
Hope this helps!