If the Power icon is missing from the Notification Area or Systray in Windows Vista, a registry change could fix the issue. First make sure you’ve tried to change the setting to enable or disable the Power Icon. If that doesn’t work, try the following fix:

1. Hold down the Windows key and press R.

2. Type regedit and click OK.

3. Click the plus sign next to HKEY_LOCAL_MACHINE
then Software
then Microsoft
then Windows
then CurrentVersion
then Policies
then click Explorer

4. Look for an entry on the right side for HideSCAPower. Right-click the entry and Delete it.

5. Click the plus sign next to HKEY_CURRENT_USER
then Software
then Microsoft
then Windows
then CurrentVersion
then Policies
then click Explorer

6. Look for an entry on the right side for HideSCAPower. Right-click the entry and Delete it.

If this doesn’t fix the issue, third party programs can cause this issue. APC software can cause the problem even after it’s uninstalled. Be sure that all power related drivers are removed from the system if you still experience this problem.