Dimension C521

I need help!! I have a Dimension C521 and have had nothing but trouble. I have had to get new speakers and a new mother board. The computer is just over two years old and of course is no longer under warranty. When I turn it on the windows screen comes up and asks you to click on your user id but I can't my mouse and my keyboard both do not work.I can turn the computer on and off with the button on the modem but I cannot get the mouse or the keyboard to work. It just all of a sudden stopped working. If anyone has any ideas please let me know.I would appreciate it greatly!!