- Start > Run > 'services.msc'
- Scroll down the list and find 'Theme'
- Restart it by clicking the link on the left
If this doesn't bring Aero back then you'll need to re-select the theme:
- Right click on the desktop and choose 'Personalize'
- Choose 'Theme'
- From the drop down list, click 'Browse' and go to 'C:\Windows\Resources\Themes'
- Choose 'aero.theme'
You should be back in business!