Yeah, just add a 2nd HDD (make sure your main drive is set to master and the secondry drive as slave if they are on the same cable). Boot the system from your first drive. When on the desktop right click on my computer and go to properties, then advanced, then Startup and Recovery and edit the startup options file. (BE VERY CAREFULL NOT TO CHANGE ANYTHING YOU ARE NOT FAMILIAR WITH). 1PL27CB001124
See link to editing the Boot.ini file
http://support.microsoft.com/kb/289022
Example
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /NOEXECUTE=OPTIN /FASTDETECT
Where 'default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS' is displayed, this means your PC will boot from Hard Drive 0 (Primary) and partition 1 - It will boot from partition 1.
If you have installed your cloned HDD as a slave on the same cable all you would need to do is to copy the 'multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect' bit but change the (0)'s to (1)'s. This means your system will now see the other HDD as a boot drive and will give you the option to boot from partition 1 on that drive.
After saying all that you could just Software Mirror the HDD's from within windows. This option will give you a mirrored drive so as the files on the main drive change so will the files on the secondry drive.
Hopr this helps