With and without www

I have installed WordPress (2.3.3) using same method at two place:

http://amritcares.com/amrit/
http://www.amritworldnetwork.com/diary/

In first link, it does not use ‘www’ in its url and in second link it does. Both sites are hosted on same server in same hosting package.

Can someone please explain why it is so? How can I make sure that ‘www’ comes in its url, please?

Thanks and regards.