Stick in an xp cd and go to run. Type sfc /scannow, wait, and reboot. Seeing as your start menu isnt working, you will have to do control-alt-delete. Then in task manager you can get into the run menu (file -> run i think) The xp cd isnt always necesarry but its recommended. SFC will check for missing or maliciously modified system files.
thank you very much for your answer, however I do not have the windows xp cd anymore. However, I still can access to my start menu and even the control panel, but I cannot open any of the programs inside it. Can I still do it without the XP CD?