I'm not scared of spiders, however, I have a general problem with small insects, which includes spiders for me (even though they're not insects ^^)
Basically, I don't like creatures that are small and light enough to walk over my skin without me noticing it.
Huge spiders, like Tarantulas, are no issue at all for me. If the insects/spiders are behind a glass or such, no issue. If I'm outside or simply see them far out of my reach, no issue.

....one single ant in my near, and I panic!
No, seriously, I have a huge phobia of ants! I can't stand them, I don't think they deserve any mercy and I'd wish I could get rid of them ALL! >.<
They're the roman empire of the insects - and I can't stand the roman empire either! (antic! roman empire, not people who live now in rome, italy XD)

And yes, my phobia of ants and my freak-out of insects I don't feel on my skin are connected!