There is a read-me file that come with wondows ce in the default.xbe folder. In there it tells you that a usb mouse is the only thing currentlys supported. so no keyboard. the box only pops up if u have the xbox controller plugged in because it is an unknown usb device.
So what u can to do
1.) Wire up a usb hub in your xbox. I installed my 4-port usb hub on the forth controller port. It was a mini stick hub about 3.5 inches long(i didnt measure) and less than an inch wide with aboue 3 inches extension of wire. And trimmed the edges and match the colors on my fourth controller port and sodered them together.
2.) Take the part of the xbox controller that plugs into the xbox and cut it and trim the wires. Then get a usb mouse and cut it off before the plug and trimm those wires. From there just match up the colors and soder them/ twist and tape them together. I reccomend sodering them.
3.) Buy a joy frag to hook up a regular mouse although i dont know if this method is supported.Pics Here