I tried this last year and gave up on it. Without using fast AD devices the results were not what I was hoping for - unless someone know a better way.

Below is a link to the project files (VS2008) that I put together for taking the usb data and displaying it on a scope type gui. It is not commented but you should be able to work out how I did it (bear in mind, Im not a very good programmer!)
I couldn't find the pic basic code but it used the excellent post by Squibcakes on usb to get the pic side of things working.

Hope you have better luck than I did

Paul

http://www.pjloughlin.com/files/usb_scope_v1.zip