I'm sorry but we need more information plus what exactly do you mean. You can't install it at all, or it is installed and you can't open it?
Please include the following information:
What is your operating system, service pack and are you up to date with Microsoft update? There are two ways of checking this. Go to Start/Run and type in winver then click the Enter key. Or, right-click My Computer in the Start Menu or on your desktop (or Computer in Vista) and select Properties. Post what it says including whether or not this is a 32-bit (x86) or 64-bit (x64) system. Plus what browser is default and what version of Internet Explorer is installed. (Help/About).
What McAfee products do you have? If your taskbar icon is like this: - double-click to open SecurityCenter and then click "View Details" or "About" at the lower right of the new window. Post the details from each module. If you don't have any installed what other security products do you have installed or did you have installed at any stage?