Yes there are loads of "how to enable Aero in Virtualbox" posts out there, but they are all long too long ;)
- Install VirtualBox > 4.1
- Install Windows 7
- Install Guest Additions
- DURING the install, manually select 3D Support
- Select Yes WDDM video driver
- Ignore the warning about safe mode and continue
- In Control Panel, Personalise re-select the Window7 Default Aero Theme.