For my website, I have set up a very basic and nooby email service. It will take the message and write it to a text file.

However, I want to set up the email so that it will send the message as an email to [email][/email].

How do I do that? Also, I am not looking for 'mailto' function..

From what I was told, it will open up a local email app like Outlook or Windows Live Mail and A LOT of people do not have that set up.

My website is running on IIS and has PHP 4.X something...

Have a look at the PHP mail command. If you do a search you can find libraries that support this and examples.