This is very easy, if you follow the advice in my books: Make a new folder for your project file each time you start a new project. If you do that, deleting all associated files is as simple as deleting the folder!
It doesn't sound like you've done that, however. So you'll manually need to track down the .prel file and Adobe Premiere Elements Auto-Save and Preview Files folders and delete them. And, if by "associated files you also mean media files, you'll need to find and delete them manually also.
Meantime, use the method in my books for your future projects, and clearing off old projects will be just a click or two away!
Thanks Steve, how do I get hold of your book giving this advice
For PrE 7 (I think that it is still in print), Amazon has Steve's Muvipix Guide to Premiere Elements: http://astore.amazon.com/chuckengelsco-20/detail/0615248993/104-3709942-5611121
If you are planning on moving to PrE 11, he has written the definitive guide to that version: http://www.amazon.com/dp/1479311200/ref=as_li_qf_sp_asin_til?tag=chuckengelsco-20&camp=145 73&creative=327641&linkCode=as1&creativeASIN=1479311200&adid=0N87W21KS3SSK1YA3MZW&&ref-ref URL=http%3A%2F%2Fwww.muvipix.com%2FFORMgallery1.php
For one Project folder scheme, see this article: http://forums.adobe.com/message/3998053#3998053 Note: this is how I set up my Projects, but can be used as a guide.
Yep, found Steve's book for Premiere Elements 11 on Amazon and bought it. Thanks both for you help.
Great news! You will LOVE that book.
I see that it took me a few extra minutes to gather the links, that Steve has memorized. That, or he just types faster than I do...
Either way, you will benefit from his book.
Once you are comfortable with the program, I highly recommend his Tips & Tricks, which has been re-written for PrE 11. He unleashes the power of the program. Though I have been doing this for a very long time, I still always learn new tricks from his books, and would not upgrade my PrE, without his books at hand. He saves me hours of hunting in the Help file, to find out were ____ went, in the new version.
Good luck, and happy editing,