So what tests have you done to eliminate what it could be?
Test the keyboard in isolation on the usb hub, does this work?
Test the mouse in isolation on the usb hub, does that work?
Try these tests on a PC too?
If any of the above tests fail it sounds like there must be some kind of incompatibility with those devices and the hub. If they succeed, but then the point of failure is when you add the wifi dongle that could be something else.
One thing you could try is to see if the USB hub is back powering the Pi. To do this unplug the Pi PSU connector and see if the Pi does not lose power. If it stays on then your hub is back feeding power to the Pi. You’ll need to cut the red wire inside the USB lead that uplinks the powered USB hub to the Pi USB ports.
You may find that if back feeding of power is going on then things will work as expected after you cut the red wire.
Here are some other people who had the power problem;
http://www.raspberrypi.org/phpBB3/viewt ... 2&p=330024
http://www.raspberrypi.org/phpBB3/viewt ... 7&p=117349
http://www.raspberrypi.org/phpBB3/viewt ... 6&p=319870
http://www.raspberrypi.org/phpBB3/viewt ... 1&p=314616