My God, there are a lot of pointless insensitive bull replies in this thread.

Getting over a phobia is a tough task. While arachnophobia is an irrational fear, it's not enough to simply prove that spiders are harmless. The fact that many spiders can cause pain or injury to humans means that there is at least some rational basis for an aversion to spiders.

I'm not a trained psychologist, though I've talked about phobias with several psychology student friends. Overcoming a phobia usually involves a slow, careful process which gradually increases a person's exposure to the thing that they fear. This is generally done in a controlled, safe environment, to help reduce the person's fear. For severe arachnophobia, it might begin with showing the person photographs of spiders for a certain amount of time. Once they can cope with that, without severe anxiety, they might move onto model spiders they can hold in their hand, or watching videos of spiders in the wild. This process continues incrementally, over quite a long period of time, until the person can finally tolerate being around actual, live spiders.
It's definitely something that should only be attempted with the help and advice of a trained professional, though. My description here is brief, imprecise and undoubtedly lacks a lot of crucial details.

Personally, I'm nyctophobic. Darkness and nighttime can, under the right circumstancs, terrify me. I'm just lucky it's a mild phobia. I very rarely get panicked, though sometimes I'll avoid going outside or into an unfamiliar building at night in order to avoid the fear. One time I had to get a friend to walk me to my car at a campsite, even though it was only a couple of hundred feet through some trees, and I'd been there and back numerous times during the day.