When I right-click Computer and choose Properties, I can see the Windows Experience Index on my Control Panel\System applet. But when I click on the Windows Experience Index link, I go to the Performance Information and Tools page, but the Windows Experience Index details are missing completely, so I can't view or update the scores. How can I get it back?

(Fig 1:) Rate and improve your computer's performance is completely blank.
This happens if a policy is set which removes access to the Performance Information and Tools page. If you're the administrator of the system, follow one of these methods below to relax this restriction:
Computer Configuration | Administrative Templates | System | Performance Control Panel
If it's already set to Not Configured, set it to Enabled and click Apply. Then revert back the setting to Not Configured. This resets the registry based policy if set already.
User Configuration | Administrative Templates | System | Performance Control Panel
Note: Group Policy Editor is not available in certain versions of Windows Vista. If your version of Windows Vista does not have the Group Policy Editor, follow the registry editing method instead, which is listed below.
HKEY_LOCAL_MACHINE \ SOFTWARE \ Policies \ Microsoft \ Windows \ Control Panel \ Performance Control Panel
HKEY_CURRENT_USER \ SOFTWARE \ Policies \ Microsoft \ Windows \ Control Panel \ Performance Control Panel
Note: If one of the above locations does not exist, skip to the next step.