Qt animated progress bar is static under Windows 7, classic style

I'm using a QProgressBar to show a device search progression. I set minimum and maximum to 0 to have an animated progress bar as busy indicator.

When deployed on Windows 7:

• If OS display is configured with using "Aero" style, it works fine, bar is "animated"
• If OS display is configured with using "Classic" style, it soes not work, bar is "static

See attached image.