Create a centered POP UP window from Flash

Hi!
I am working with a website in Flash MX 2004. I want to have a centered POP UP window when someone clicks on a menu button. I have tried out some actionscript that I found here. It works well, when it’s in the first frame of my timeline. The site I work with the menu is a bit inside the timeline. Now when someone clicks on the button a window pops up. But it’s not in the correct size that I wrote in the script. It’s much bigger. Why is that? How can I have a menu button working with a pop up window that shows an external html file?

Please help me out! Thanks!

Best regards
Joakim Norlin
Malmo, Sweden