Or you can download and use the free version of easyHDR (High Dynamic Range).
You enter the different length exposures and tell it to stack. Much the same as Registax, but it layers the bright and dark accordingly giving you a final image of everything except burnt out bits
Very easy to use, even I can use it
The 'easyHDR BASIC' is the free one:
http://www.easyhdr.com/download.php