Hi Phani Kumar,
Thank you for this report.
We are aware of this issue in the RadRibbonWindow control working in Windows 8 OS.
And it is already fixed in our latest internal build(s). This fix will be also included in our next official releases.
You can try our latest internal build, where this issue is resolved.
Please, let us know if you still experience any problems with the RadRibbonWindow control.
the Telerik team