Silverlight 5 malfunctioned?

Why is the CPU usage so high even hardware acceleration is enabled in Silverlight 5 (64-bit)?
(VGA: ATi Radeon HD5750)

Windows 7 has a different way of allocating resources (whether CPU, or RAM or something else.) The philosophy is "unused resources is wasted resources." So unless you're experiencing some specific issues with Silverlight not working as expected, the CPU usage probably isn't anything to worry about.

You could click on the Processes tab in Task Manager to see which process is hogging the CPU. It could be something else entirely that's taking CPU time away from Silverlight. Click on "CPU" at the top of the column and you can adjust the order in which the processes appear (most usage last, most usage first, etc.)

If it is a Silverlight issue you could try changing its priority. Click the Silverlight process to highlight it, then right-click "CPU" at the top of the column. Go to "Set Priority" and you can change the Silverlight priority from Normal to AboveNormal or High to see if that makes a difference. You'll probably get a warning that changing the priority could cause system instability. Up to you if you want to see if changing the Silverlight priority makes a difference in performance. You could always change the priority back to Normal if there are issues.

And if you suspect that Silverlight has become damaged or corrupted in some way, this Microsoft KB article may help.

How to clean a corrupted Silverlight installation and then reinstall Silverlight