Problems with starting PHP. HELP NEEDED!

[mr02077]

        I installed PHP, MySQL and Apache on my computer. I lunched the page with phpinfo() in my browser and it works.
        Problem is: every time I start php file directly from browser it starts correctly, but when it should be called from some HTML file that I opened in my browser it doesn't work. Infact before I installed PHP i accidentaly clicked that Windows should always open php files with Notepad. And in described situation he opens it with Notepad. If I change the programme that Windows should use it tries to open it with that one.
        I don't know how to solve this problem (or to undo the Windows opening programme selection, or wich programme to choose).
        I appreciate any help. Thanks.
                    Dusan, Serbia