about: adverb or preposition

Hi,

I'm new to this forum. I'm wondering if 'about' in the following sentence is an adverb or preposition:

I was about to cry.

I saw the sentence in Webster's New World Children's Dictionary, which puts it as adverb, but Webster's New World Student's Dictionary puts it as a preposition. In both, the definition given is 'almost ready'.

Thanks,
Clarence