You can't just open a single page and have changes reflect in the site itself. In fact, if you make changes to the aspx page without recompiling the code behind afterwards, you will likely break the page completely. At minimum, you will have to open the project associated with the assembly that contains the page you are trying to edit. Once the changes are complete, you will need to rebuild that project. Then to debug the solution you will have to open the page in question using the browser, and then connect to the running process from Visual Studio 2003( Tools->Debug Processes Menu, then attach to the aspnet process). Set relevant breakpoints in the code and away you go.
If you are having issues with getting IIS working properly on an IIS7 install, this link may help:
http://blogs.iis.net/bills/archive/2008/06/02/installing-asp-net-1-1-with-iis7-on-vista-and-windows-2008.aspx[
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[Update]
After rereading your comments to the previous answer, you would need to right-click on the page you wish to edit in the solution explorer and select the View Code option from the context menu that pops up.