First off, don't use frames unless you ABSOLUTELY HAVE TO. They will cause headaches, I assure you.
Second, I would suggest loading a user control as opposed to a form. You can define ContentPlaceHolders which allow you to dynamically load controls in to them as needed. The place holders themselves will be rendered as divs, so keep that in mind.
As far as the frame thing goes, you can probably do what you need with a couple of floated divs. Look up the "float" CSS attribute.