As we say in my books, the smartest way to delete a project is to create it first in an easily deletable way.
In other words, whenever you start a new project, open a new folder for it. That way, all of the project files, captured video, rendered files and scratch files will be in one place. Deleting the project is as simple as deleting the folder.
Otherwise, you can deleting the project file manually as well as any supporting video clips. You may also want to delete the Auto-Save, Preview, Encoded, Layout and Media Cache files for that project within each of their sub-folders -- though you can also leave them. They'll take up space, but they won't hurt anything.
Thanks Steve, I'll remember to use a folder in future, but how can I delete what I already have?
As I said, you'll have to manually delete the projects .prel file as well as any video clips you've captured for the project. Then it's up to you how deep you want to go cleaning up scratch and render files.
You'll find a sub-folder of Adobe Premiere Elements Auto-save files that will be about 30 meg. Make sure you only delete the ones for this project!
You'll also find a sub-folder of Adobe Premiere Elements Preview files, one for Encoded Files and one for Media Cache files. Inside of each will be a sub-folder named after your project that you can delete.
See how much easier things would be if you'd have read my book first?
What Book? Tell me about it.
I've published a number of Muvipix Guides to various Premiere Elements versions, including the Photoshop Elements/Premiere Elements 7 combo. YOu can find them by doing a search on my name on Amazon.com or you can order them from the Muvipix.com book store.
Along with Steve's suggestions, one thing that works very well for me is to basically only use copies of my Assets in my Project folder structure. This allows me to just delete that Project's root folder, and clean out everything, but does not touch my original Assets, as I only have copies in the Project's folder structure.
One consideration is with Captured footage. If one has the original miniDV tapes, one can re-Capture at a later date, so whether the original Capture files are in the Project foder structure is less critical. I will Capture to the Project, and then make a Copy of those Capture files onto an external HDD for potential later use, without the need to re-Capture, even though I retain all tapes. I'd rather do this, than spend the time to re-Capture and the tapes are just another level of backup.
If one is using source footage from a non-tape source, things can be a lot more critical, as flash media, internal camera HDD's, etc., will be reused and overwritten. For these, I would definitely make a Copy to an external for archival storage - no question about it for me.
When done, I also use the Project Archiver (Project Manager in PrPro) to make a complete copy of all files to an external, in the event that I need to go back and make corrections at a later date. Steve's books will cover the Project Archiver in detail, and you might find it very useful for getting you all materials needed for later editing.
I use externals a great deal, and few Projects do not get archived to these, along with the originals (or Copies) of the Assets.
Just some additional things to think about for the future.