default_controller problem

[eluser]Santiag0[/eluser]
Sorry, but didn't work too. I try with some variants and always show error 404. I think that is it because always try to load a class, if I put:
http://localhost/
Show welcome
If i put
http://localhost/usernameX
Try to load a class, so display "404 Page Not Found"

I think that the solution its put a code to modify all urls, in this way:
If ($this->uri->segment[0]) != 'go')
{
$this->uri->segment[2] = $this->uri->segment[0]
$this->uri->segment[0] = 'users';
$this->uri->segment[1] = 'name';
}
else
//this->uri->segment[0] is equal to go, so the second is the correct class, the third the method, etc...
array_shift($this->uri->segment); //Shift an element off the beginning of array

When i can put this code?

Thanks a lot for your help, Big Student!!