Wordpress in subdirectory

[pyramatboy]

Hi Guys,

I hope this is the right place to post this.
I have an existing site and want to put wordpress in a subdirectory of the site to run a separate section of the site.
I have created the subdirectory folder and installed and extracted wordpress there.
However I am then getting an error regarding the wp-blog-header.php when I try and go to that folder. I then read that I need to edit index.php with the location of the subdirectory. I have done this but it still does not work.
Then I read that I need to move index.php to the root directory, however I can not do this as I already have an index.php file there for the main site.

any ideas here?

thanks
Marc