I've always had this problem, even after the fresh install of Win7x64 when first built.
I use a Logitech wirelsss keyboard/mouse in one of the rear USB ports and that works fine. Often, not everytime though, when I plug in my webcam into a front USB port (off the front UDB header) I loose the keyboard and mouse. Sometimes the same happens with a hard drive with a USB interface.
Another thing that happens regularly, but not everytime, once one item is plugging in the front a second device won't be seen, not even unrecognized, it's like it isn't even there. The multiple devices off the front header can be anything as lowly as two microSD cards.
I've played with drivers, different device combinations, and looked at settings without a resolution. So, I have got to suspecting the mobo, DP55KG, and maybe it's USB support or the USB power supply of the mobo.