APDivs or Absolutely positioned divisions are absolute. This means they are positioned relative to top left corner of the browser viewport. When you resize the viewport, they remain right where you put them. Everything else around them moves.
What you need to know about APDivs
Best to use default CSS positioning (which is no positioning at at all). Add margins, padding or floats to align elements on your page.
See Basic 1-Column CSS Layout (view source in browser to see the code).
Learn CSS Positioning in 10 Steps
Am I better off slicing the image and sticking the form in where I want it to go? I have done that in the past with photoshop and Adobe Golive. Will that work with Dreamweaver too?
Sure. Have a look at the following Tutorial:
Taking a Fireworks (or Photoshop) comp to a CSS based layout in DW