I have an old Nikon D80 with me. When I shoot with higher ISO, like 800, vertical color bands are produced in the image.
Why does it happen and is there any way to eliminate it?
Image taken at f/29, 1/160, 44.00mm
Based on what I've read, CCD's are prone to producing excess noise with high ISO and long exposure. The effect you are observing may be blooming or smearing of the CCD. According to Wikipedia:
When a CCD exposure is long enough, eventually the electrons that collect in the "bins" in the brightest part of the image will overflow the bin, resulting in blooming. The structure of the CCD allows the electrons to flow more easily in one direction than another, resulting in vertical streaking.
Avoid the problem by avoiding high ISOs and long exposures.