Well, you can improve slightly with softwares like NeatImage, NoiseNinja, Photoshop; etc. However, the use of software isn't majic cure. There's a saying 'trash in - trash out'. It means there's nothing you can do about a hopelessly noisy image.
In Photoshop you go to the menu: Filter - Noise - Reduce Noise... Then you just have to find the settings to your liking. Another thing is that, when you reduce noise you will also lose some details. There's a compromise.
Nikon D90, D80
Nikkor 16-85mm AF-S DX F/3.5-5.6G ED VR, Tamron SP AF 28-75mm F/2.8 XR Di LD Aspherical (IF) macro, Nikkor 50mm F/1.4D, Nikkor 50mm F/1.8D, Nikkor AF-S VR 70-300mm F/4.5-5.6G IF-ED, Sigma 105mm F/2.8 EX DG Macro ||| 2x SB800 | SB600 ||| Manfrotto 190XB