She really could be purebred. Especially considering her age, she looks like the old style arabs. In fact I've seen many purebreds that looked extremely similar to her (so much so that I wouldn't doubt similar breeding). If she was conditioned, cleaned up nice, those arabian features would pop out quite a bit more.
She very well could have some other breed in there, but I would guess it's a small percentage.
Did you get her from her breeders? You might ask them (if you haven't already) her parents names, and if they were registered.