You access Control Panel by clicking on Start you should find it on the right column.
So other people can't mess with your computer. The same reason why you have a separate password to lets say Facebook or your e-mail.
This may be because you have a virus and the virus/trojan has restricted you from accessing this to solve the virus. Effectively, the virus will stop you from getting rid of the virus if that makes...
Control panel>Administative tools(switch to classic view if you are on Vista)>Computer Management Disk Management is under Storage
Click on Start---->Click on Control panel in Windows XP.Click on Windows logo in Windows 7 then Click on Control Panel.