Permalink and .htaccess ?

I posted this yesterday and had no relies. I tried to add to the original post but is says the its no longer active. PLEASE don’t remove this I really need the answer.

I am trying to change the current permalink structure of my WordPress website from;

/index.php/archives/%post_id%

to

/archives/%post_id%/%postname%/%category%

I have tried changing the permission on .htaccess to 666 for this purpose but once everything is saved and I try to visit other pages, there’s an error message, saying page can’t be found.

I realise I need to change something somewhere else, but I’m not sure what, as I thought the .htaccess was the trouble.

Any help would be great thanks.

Ashley