It's been a while since I had to use Fiddler with Cassini, the Web server included with Visual Studio. Fiddler is a HTTP Debugging Proxy which logs all HTTP traffic between your computer and the Internet. It is free and it is a must have tool.
However, to use it with Cassini, you must do one little simple thing: in the URL of your browser, you must add a dot (.) after localhost. Something like http://localhost.:5555/Default.aspx
Notice the dot: http://localhost.:5555/Default.aspx
Get Fiddler here: http://www.fiddlertool.com