If you should run an application that is incompatible with the Windows Aero color scheme, it may be automatically turned off during execution of the application. The visual elements disabled in Aero should be automatically enabled again when the program exits, but if it does not. Here is how you can manually choose Windows Aero as you scheme:

Start > Control Panel > Appearance and Personalization > Windows Color and Appearance (you may have to click "Open classic appearance properties" if the Appearance Settings dialog is not displayed). Select "Windows Aero" from the "Color scheme" list and then click the "OK" button.

Note that if Vista is disabling Windows Aero due to execution of an application that cannot support it, you should be notified with a dialog like the one below.

Text: "The color scheme has been changed. The following program has performed an action that requires Windows to temporally change the color scheme to Windows Vista Basic."

In this example, I really cannot tell what application is the problem so you may have to pay attention to what you are doing. In this case, I was remote controlling a machine with GoToMyPC. When doing so, the target system to be controlled displays this message (at least until the client is eventually updated to support it).

Note that some applications may have a work-around so check out the application vendors site. Sometimes video settings offered within the application can provide a way past the problem such as with the Zoom Player where a workaround is possible by opening "Options / Setup" and in Basic Mode: Audio/Video where you can set "Video Renderer" to "VMR7" or "VMR9" (instead of "overlay mode").