Try the following:

1. Right click on the shortcut (START > All Programs > Captain Sim > 777 Captain (Prepar3D v3) > ACE) and select Run as administrator from the list that pops up (it should be the 2nd one down from the top).

2. Right click on the 777 Captain Base Pack installer (csz772_1700.exe), and select the Run as administrator option. Select the Repair option. It might be worth turning of your Anti-Virus program, or the shields, before installing.

Note. If the first option gets it working correctly, right click on the ACE shortcut, select properties from the list that pops up (it should be the one on the bottom), select the Compatibility tab, look under Privilege Level, and checkmark (place a tick in) the box beside Run this program as an administrator, then click OK. It will always open with administrative privilege.