2000 Ford Mustang Engine is running rough and check engine

My 2000 Ford Mustang has been sitting in the garage for about three months without being driven. I have started it up from time to time to be sure and keep it in running order. However, this last weekend I started it up and the engine began running rough and the check engine light came on within the first minute of starting it. Do I need to take into the dealer or mechanic to have it checked as I don't drive it very often any more or should take it out for a good run?

I don't want to get stranded if I do take it out on a run with the engine running rough and the light on. Let me know what you think. It has been running fine all along and the last time I drove it it was running fine as wel.

Thanks,

Aaron Armijo