GUI checkbox question

Hi Guys,

I have created my first form with Koda and don't understand a couple of 
things:

<snippet>
$Checkbox7 = GUICtrlCreateCheckbox("Backup Archive (only required after 
Archive)", 51, 212, 233, 17)

$Checkbox8 = GUICtrlCreateCheckbox("Delete Archives and recreate  ", 51, 
262, 233, 17)
<snippet>

Basically what the code does is to give the option to recreate the rar 
archives from scratch, which sometimes gives a significant size saving.  
If this is done I need to force a recreate of the archive rar file.

Both checkboxes are unticked. How do I "tick" checkbox 7 automatically 
if checkbox 8 is ticked manually?

Also can't get my head around GUICtrlSetState in general:

<snippet>
$Checkbox6 = GUICtrlCreateCheckbox("Backup Bacs Files", 51, 182, 121, 17)
GUICtrlSetState(-1, $GUI_CHECKED)
<snippet>

This ticks the checkbox, but what does the -1 do?

Sorry for being so dim!

Steve