I just updated Android SDK to version 21 and trying to start emulator. I am using Nexus 4 with CPU Intel atom (x86) 768 RAM and 524 internal storage. Emulator starts fine but it
REinstal the emulator..
Go to your AVD location and delete all the files in avd folder( C:\Users\acer.android\avd ),
then Go to android studio AVD Manager and install a new emulator.
( but this will erase the current emulator so, you'll need to download it again..)