Reducing 'Noise' in the photos?

I use nikon D40 & the lens is Sigma 70-300mm. I usually take photos in ISO 200 and use a UV filter. I get fair amount of noise in the photos. According to what i know if you take photos in low ISO there should be no noise. What are the factors which determine noise and how can i reduce it.

The Sigma has a very deep lens hood so the uv filter does what?

An example or two would have helped a lot. What can cause noise is if you adjust exposure a lot in processing or if you under expose.

All digital cameras have some noise that can be seen in shadow areas or underexposed areas of the image.

When was the first time you noticed noise?

Adobe Lightroom has a feature that allows you to reduce the noise when processing your images. Noise Ninja makes a product that can reduce noise as well.

Here is a sample of the noise you might see when shooting at high ISO's

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This is acceptable noise when shooting under low light and when the image is destined for a newspaper or magazine.

Here is a link to some shots taken with a Nikon D40. How do they compare to yours, noise wise?

http://www.sportsshooter.com/members.html?id=4135

Expose the image properly because if you have to go back to correct an underexposed image, the noise will become a lot more obvious.

as it was mentioned before, adobe lightroom has a great noise reduction filter. You can download a free trial from adobe.

Expose your image properly - and don't bump up the exposure in post processing, as that usually makes the noise worse. You could remove the noise with image editing software. And there are dedicated noise reduction plugins (and standalone applications) you can get like Neat Image http://www.neatimage.com/

Using a UV filter will have no effect on noise.

Taking images in low light situations usually makes noise worse, so if you have adequate lighting, this should also help improve the situation. As you said previously, bumping up the ISO also makes noise worse.