If you want to solve this problem you can do it in two different ways:
Firstly you can use iframe instead of the content place holder, but it's not good solution for asp.net that provides so many web controls - it's solution for begginers who use html.( I have used this way until I started to learn asp.net and started to use master pages.)
The better way how to make the scrollbar is to put the "ContentPlaceHolder" into web control named "Panel". Then you can choose where you want to have the scrollbar. Don't forget to set width or height of the panel! Use it at the same position in code as "div" tag in the previous post.
Please write here if you'll choose one of my solutions. I think that the second one is better than klaus's :-) But it's just my opinion ...
Hope that this post helped you. Don't forget to click "Mark as Answer" on the post that helped to solve your problem!