Asus has a Macbook Air killer in its hands.  If there is any laptop that can compete head-on with Apple’s slimmest, it’s the just unveiled Asus UX21. At the recently held Computex 2011 event in Taiwan, Asus revealed its latest ultrathin laptop with 11.6 inches screen and measuring only 3mm at its thinnest part and 17mm at its thickest.  This is the practically same measurement as the 11-inch Macbook Air.  In terms of design and finish, however, the Asus UX21 has what it takes to beat the MacBook Air.  The entire body of the Asus UX21 is built from aluminum alloy and it weighs only 1.1kg (2.4lb).  The trackpad of the UX21 is made of glass and the keyboard keys are all metal. I know I shouldn’t be judging a notebook by its cover but with the Aus UX21, I can’t help but judge the notebook purely on looks.…