If you have a WDDM capable card, by default Windows AERO should be your
default theme when you first log into Vista. If it does not, either you do
not have updated drivers installed for your video card, you don't have
minimum recommended amount of vRAM which 128 MBs. Could you give us some
info on what type video card you are using?
To enable AERO, right click the Desktop > click Personalization > Themes >
Windows AERO. If its not available but you are sure your card is supported,
the Desktop Windows Manager Session under Services (Administrative
Tools\Control Panel) has stopped.