In my book I recommend that, whenever you start a new project, you create a new folder for it. That way, all of your captured video, project files, rendered video and scratch files are in the same folder. Removing it all from your hard drive is as simple as deleting the folder.
If you didn't do that, things get a bit more complicated. Once you've backed up what you want to keep, you can manually delete the captured video, rendered files, etc., from the sub-folders -- but you'll need to do it manually. Hopefully the files will be recognizable by the names you gave your project and capture files when you created them.
Such a simple solution. Will do!
Again, depending on your initial Project layout, Archiver will likely help you.
I do pretty much as Steve suggests, and always have. Each Project becomes a root folder, with everything inside of its hierarchy. Most Assets, that are not Captured, are but copies for various storage folder, and all Scratch Disks are located with my Project. It makes this last step - archiving, so very easy.