If I goof up an install how can I remove the right files & start over?

I’m totally new to WordPress but did an okay install through CPanel Fantastico (wow pretty hard huh?) and it worked fine. After doing a couple blog posts I tried to update the theme. I had not installed WP on my main directory but put a “blog” directory in the public.html file and installed it there. Once I tried to update the them the “blog” folder is totally gone and I can’t get to anything through the wp-admin even. I think I’m so far gone that I probably need to just delete all the folders that were installed by the Fantastico in my original install and then start over. How can I find which folders to delete without deleting something from my site that shouldn’t be?
Any suggestions for a newbie?
Thanks.