Domain woes

I dunno what could possibly be causing this problem, as far as I’m aware. all the required DNS. .htaccess etc etc configs has been applied…

When I go to http://www.domain.com, it shows my wordpress homepage as it should do, however when just typing domain.com into a browser bar or http://browser.com it forwards me to the wordpress signup php page with the username domaincom filled in manually.

What do i need to look at so that domain.com in the URL bar goes to the www side to show my homepage and not the signup page?

As another question; if a user goes to a subdomain that isn’t used, it comes up with the signup page – I don’t mind this, but the username box is for some reason adding the domain.com part to it, for e.g if a user goes to nosignup.domain.com the signup username box defaults to nosignupdomaincom