If you want to use the spare, for your own peace of mind have a tire shop take the tire off the spare and check for corrosion, clean it and then remount it. The tire even though old shouldnt be in terrible shape. It's located in the trunk unless you have a water leak or holes in the spare tire compartment then it should be good to go.
I believe (i'm sure some will correct me if i'm wrong) but you can have up to a 5% tire diameter difference within your tires before it would become a problem with your AWD. As far as the tread goes, even though the spare has more tread then your current tires have, I doubt you will be exceeding the 5% diameter difference.
Last edited by E46er75; 09-17-2012 at 08:38 PM.