I'm wondering if it's possible and if it is then how can i customize KDE like this..

Here is the default Windows Start Menu:
http://www.mydigitallife.info/wp-content/uploads/2007/08/vista-start-menu.png

And here's the WoW themed one:
http://i40.tinypic.com/2w3uu5u.jpg

How can i customize KDE like that?? I mean i don't need WoW theme for KDE just to know if i can change KDE that much..

In KDE 4.2 the kickoff menu is rendered via the current plasma theme. You can read here how to create such themes.