So I'm hoping you can help me settle a bit of a disagreement between myself and a friend.
Okay, background:
My friend is a strict Muslim I worked with in the past. We used to have many interesting discussions on religion, comparing notes from his background as a Muslim with mine as a Wiccan and Buddhist ... of sorts

Anyway, he told me once that Islam teaches that there were over 120 000 prophets before Mohammed, and that, by his understanding, there were prophets in other regions of the world as well, so that, conceivably, Buddha, Quetzalcoatl and all these other enlightened figures in myth and legend were actually prophets sent by God.
I have a somewhat similar belief in that I think that, whether they are sent by God or are simply remarkably wise, gifted individuals, there's a teacher every couple of centuries or so that kinda ... "reminds" us of the important stuff, and what our purpose really is, namely working towards enlightenment and living a just, compassionate life. This stems from my belief that, if there is a God, (s)he wouldn't just reveal hirself to one group of people but to everybody around the world, and that they would interpret that revelation according to their cultural traditions and circumstances, and that these teachings, though initially as pure as a human oracle could interpret it, would eventually and inevitably become corrupted by ambition, politics, translation and time, necessitating these "top-up" teachers, if you will.
The disagreement came in with me saying that, assuming all these prophets were sent by God, that would mean that all of the traditions are valid paths to God, only that some are more corrupted than others, and that you don't necessarily have to follow Islam as laid down by Mohammed to eventually end up in heaven or attain enlightenment. He maintained that Mohammed's teachings supersede all previous prophets, and that Islam is thus the only valid path.
So from the point of view of Islamic teaching, which position is more correct?
Mina.