I am working on a Dimension 3100/E310 and it only has the option of USB for keyboard and mouse.  I plug a USB mouse and keyboard into the system and can get into the BIOS fine, but when Windows XP Media Center Edition loads to the log on screen the mouse and keyboard will not function and the keyboard number lock light stays on and doesn't respond.  I have tried unplugging it and plugging it back in but then it won't light.  Any ideas what is causing this?  I have had this happen multiple times with this series of desktop.