Auto fill series option as default?

Hi all, Excel 2007 user here. When i'm at work (this may be Excel 2010, i can't remember), i only have to enter the value 1 in a single cell and i can drag down the cell and it will autofill as a series instead of just copying the value 1 all the way down. when I'm at home (Excel 2007), i have to hold the control key as i drag down in order for it to autofill as series. why is this? could it be a difference between Excel 2007 and 2010? Is there a way i can set it so that i don't have to hold down the ctrl key and Excel will by default autofill as series?

i realize this is an incredibly nitpicky question, but i am just curious. thank you.