Toad for Oracle 64-bit : 1603 - Fatal error during installation
I get this error all the time with the installation or updgrade of the sql optimizer tool, For me this error is always due to the required VBSCRIPT.DLL and JSCRIPT.DLL files not being correctly registered.  Following the step listed here:
http://social.msdn.microsoft.com/Forums/en-US/jscript/thread/b7d1a35f-3759-4217-91ba-e4416ac19d78
works for me.