I'm going to take the opposite opinion from the others, I wouldn't write. This is a girl you knew for only a year or two (because that time was split up), 18 years ago, and now you've tracked her down on the internet?
And not on Facebook or MySpace?
That sounds creepier than Speeder Mike's story.
I know you said you weren't interested in her romatically but the fact you mention that makes me wonder if that is the reason you're looking to talk to her. I've known and lost touch with a lot of people over the course of my life and out of all of them, there's really only 1 that I ever think about anymore and it's because it's a girl I was interested in but never made a connection. I still wonder "what if" sometimes.
Matter of fact, my 20 year high school reunion is this weekend, I'm not going. I have no interest in anyone that I went to school with, and I'm from a small town and spent 13 years with them. My close friends from high school, I still keep in touch with them, the rest, no interest.