On a 64bit OS, you may not be able to modify a .txt file with Text File Changes in the System32 folder. This is due to the .dll being used by the ISSearchRepaceInstall and related actions.
Discussion
To get the Text File Changes to work for files in the System32 folder on a 64bit OS you'll want to make sure the Template Summary is x64;1033
Make the component containing the Text File Change a 64bit component.
Then you'll want to go to the Custom Actions view->Execute section of Installation Sequence and find the ISSearchReplaceInstll action and set the DLL File Name to: <ISProductFolder>\Redist\Language Independent\x64\ISSchRpl.dll
When the info box prompts: Other custom actions use... Do you want to update all custom actions that use this path?
Select Yes and it will change the other actions that need to be modified with the correct path.
This will force the action to use the 64bit dll which should then be able to modify files in the System32 folder.
