Blurry images are often caused by motion blur. Does your camera have image stabilization. Even if it does, shooting at 800 ISO without flash indoors with normal lighting is likely to cause blur...especially if your aperture is anywhere above 4.
Do you know the settings for that shot? I can help you more if you give me some info about it. What distance were you from the snake? Were you zoomed in all the way, or zoomed out for a wide shot?
WHen you zoom in, your minimum focusing distance increases. So if you try to get really close, and also zoom, you will never be able to focus. When I do a macro, I zoom out all the way, and then bring my camera as close as it will allow and still focus. Do you get a green indication that it has focused?
A tripod always helps, as does a timer delay on the shutter. Pressing the button to take the picture is one of the main causes of camera shake. Using a 2 second timer will give you 2 seconds to recompose the shot, exhale, and hold PERFECTLY still. You can't always capture the emotion you want due to the delay, but you can usually eliminate motion blur.
If you can supply the info below for any of your images, I may be able to tell you what happened, and what you can do to get better shots.
Camera Model
Shutter Speed
Aperture
ISO
Focal Length (were you zoomed in, or out)