Dell Optiplex 745 usb keyboard and mouse will not work after windows loads

The problem I am running into is that the keyboard and mouse on two of the computers on my network will not function. The keyboard works just fine during startup, I can get into the BIOS and all that happy hooplah, but as soon as windows loads up they refuse to work. They are getting power from the computer but the computer refuses to use them. I would try using the PS2 keyboard port however with these new “fancy” dell’s there isn’t one. I need to get these machines fixed and everywhere I look to try and find an answer to this no one knows. Please let me know what you guys think.