Epic win. First, download the free version of Visual Web Developer 2008 here. Install it. Enjoy waiting twenty minutes. Get a cup of coffee. Don't sit there waiting for the worst out of box experience ever, whatever you do. Next,
go to Internet Explorer's Internet Options and set Internet Explorer as your default browser. I know, terrible, but you gotta do it -- otherwise VWD2008 tries to use Firefox, Chrome or your other superior default browser and you lose the ability to debug IE7 in all of its awful putrescence.
While you're in Internet Options, you should also de-select disabling script debugging and enable debug messages in the Advanced tab. Now
you have to create an Empty Web Project by clicking New > New Website. Now click Debug > Start Debugging. It'll open a new IE window pointed at localhost, but just fire up whatever to-be-debugged website in that window. Awesome.
Now any errors should drop you into the VWD2008 debugger. ROCK ON.