Changing a script example to use a said Drive letter from drop down

I am requesting some guidance. I am modifying an example script located here:
http://www.autoitscript.com/forum/topic/125610-windows-user-files-back-up-and-restore/

I have drawn the GUI and added some directory paths. I now want to change the script to add a drop down box to select an external drive letter. Is it possible to just create a variable and place the variable from the selection into the script everywhere the C:\ is stated? If so can someone give me an example?

Thanks in advance.