[Help] Login error message for each user

Hi to everyone,
I’m using wordpress to enhance and develop my first dynamic website.
I managed to customize it a bit, and i’m having my first troubles.

I would like to make users can register at my website in order to give them a personal page and personal utilities.

I downloaded REGISTER PLUS and AUTHOR AVATAR LIST widget.

When the user make its login, this message occur:
“

PHP Warning: require(./wp-blog-header.php) [function.require]: failed to open stream: No such file or directory in C:\WebSites\orchideina.com_cj8tjsiwqmo054jw\wordpress\index.php on line 17 PHP Fatal error: require() [function.require]: Failed opening required ‘./wp-blog-header.php’ (include_path=’.;C:\php5\pear’) in C:\WebSites\orchideina.com_cj8tjsiwqmo054jw\wordpress\index.php on line 17

“

Going back to the previews page, or refreshing the page in the browser, the website appear normally and the user result logged in.

How can i avoid to make this message occurs again? What’s wrong?

Another question: How can I make that each user have a personal Avatar, which he can select from a default list or a personal link?

the website is: http://www.orchideina.com

Thanks in advance.