I am using windows xp and I cannot open the control panel from the start menu.

What happens when you try?

Reboot your computer into SAFE MODE and see if the Control Panel is accessible now. Make whatever adjustments you wanted, then reboot normally.

If it is not seen in safe mode try going to the start menu and clicking run. In the run box type sfc /scannow but make sure you have the xp reinstallation disc available because it will be needed.