If you try to install a Windows 7 system in a Virtualbox you will notice that there is no 3D support there - no feature of Aero works and the Desktop is shown in the Aero fallback mode without transparency.
But if you have the latest version of Virtualbox installed there is something you can do about that - experimental 3D support is now available with the help of a guest addition. To install it, open your virtual Windows 7 box and select Device > Install Guest Additions... to start:
Wait a second for the next window to pop up and click Run VBoxWindowsAdditions.exe:
Follow the setup until you can choose what additions to install. Select Direct 3D support (experimental). As it says, it is experimental so you shouldn't use it on production systems - make sure to have backups made of everything important:
When the warning appears, don't click on Yes blindly - that will make you cancel the installation. Read instead and click on No:
Follow the last steps of the installer and restart your machine to find Aero working. It may lag at times but that is what you would expect in a virtual machine with limited resources: