I'm not sure I'm reading this right, but if the product selection is a form then you should be able add a target attribute to the <form> as
ADAMWebDesign showed for a link.
If you go the JavaScript route then people will need to have JS enabled on their browsers so that they can use the sign up form, in which case:
a) How are you going to deal with those that choose to have JS disabled?
b) More importantly how are you going to meet Guideline 6.3 of WAI (
http://www.w3.org/TR/WAI-WEBCONTENT/#tech-scripts) which requires pages work without client side scripting.
If you can't use frames the way
ADAMWebDesign described, wouldn't it be better to use server side scripting to achieve the functionality you require?
Sites are supposed to be accessible now days.