Alt+Tab change application, system hang-up!

[mintweike]

system : Linux Mint 12 (Lisa, Gnome)
Hardware : Dell Vostro, 2Gbytes Memory
Software : Oraclel VM VirtualBox -Version 4.1.14_r774400, Firefox(Lisa default installed)
Steps To Reproduce:
1. Install a Windows7 on VirtualBox with memory 1024MB, and then, keep Windows7 running!
2.Open firefox, and open some websit , keep their tag!
3.Open file system using, or other applications
4.Now you press "Alt+Tab" to change one application to another,
5.Screen will show a square area for you to choose which applicaton you shall change to !
   Some times, System will hang-up here, and can never back to desktop!
Happen Rate : 80% (I tried 10 times!)