This happens mostly on 64-bit machines. These machines do not install products to ‘Program Files’ but rather to ‘Program Files (x86)’. When you are running the update, you can use the browse button to point the update towards the correct location.
You can also check the location, by right-clicking on the ‘PT’ icon on your desktop and selecting ‘Properties’. The ‘Start in’ location must match where you running the update to.