There have been many cases of autorun.inf being used in conjunction with USB drive infections, these often put a copu of autorun.inf in the root of any drive or partitions to continue with the replication of the infection.
Normally you don't see a copy of autorun.inf on a hard disk, this is more removable media and usb drives, so it may be because it is on your hard disk that it has been quarantined. Unfortunately I know very little about the workings of combofix to say exactly why it did that.
I believe the only reason you have a copy is because of the recovery partition, this would I assume act like an installation CD and that would have an autorun.ini file.
A search of my system only finds one and that is in a Folder back-up copy of XP SP2 update that I copied of my CD.