Aero is normally not available on Windows Vista Home Basic and Windows 7 Home Basic. Furthermore there is an internal hardware check which only allows Aero to run on a system if passed.
This tool can enable Aero for you and additionally disable the hardware check if required.
NOTE for Home Basic users: Enabling the Aero feature does not mean that transparency will be enabled. Only Aero taskbar thumbnails and maybe some other features may be enabled by this utility. There is no way to enable transparency on Windows Home Basic.