How to re-enable aero effects in Windows 7 after it has crashed?

Restart DWM with (by using a CMD shell window with Administrator permission):

net stop uxsms
net start uxsms

It's the same service that crashes and is also known as the Desktop Window Manager.

"taskkill /f /im dwm.exe" always did it for me, though Diago's answer is cleaner. Killing the process should get the DWM's helper service to restart it pretty much right away.

For me, when that happens I also have a wrong screen resolution. After I set it to the right resolution, in the Settings I can chose Aero as the Design again.