Change the batteries message keeps appearing even with fresh batteries. The camera turns on then immediately turns off What's up?

This actually a really common problem among Canon point and shoot cameras. There are a couple of methods that have worked well for others.

1. Open the battery compartment and remove the batteries. Then take a regular wooden pencil and place the eraser end down onto the battery contacts. Rotate the pencil to remove any corrosion. Sometimes a lack of proper battery contact will result in the low battery indicator.

2. Take a toothpick and wedge the tip under the battery contact and break it off. This will put more pressure between battery and contacts.

You can also try installing 3rd party firmware at http://chdk.wikia.com/wiki/CHDK_in_Brief
This is a bit technical and will probably void your warranty, but might be worth the try.

My problem happens when a freshly charged set of NiMH batteries are installed. The fresh charged voltage (1.36v/cell) makes the camera think you have a set of Alkalines installed. When turned on, the current draw drops the voltage slightly, hits the Alkaline "low voltage shutdown limit", and turns off... Need to slightly discharge the batteries to lower the fresh peak charge voltage, camera will then see the batteries as NiMH rechargables, and use a different voltage profile. Works for me !!!