Assuming this is a cathode ray tube type TV?...
Unless it's received a substantial bump (eg. dropped), then it's likely it's just a magnetized shadowmask, causing beam landing (purity) errors.
It's usually simple and inexpensive for an engineer to degauss it, or repair (if due to component failure, eg. PTC Thermistor).
http://en.wikipedia.org/wiki/Degaussing#Degaussing_monitors
If you have any loudspeakers or other devices with strong magnets close to the TV, try moving them away.
If a source of magnetism is removed, often the fault will correct itself with normal use.
If it's been dropped then you may have dislodged the shadow mask or tube yoke, which would be expensive - but fortunately, that's very rare.
Cobweb
February 2007
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Try fixfault.com to fix televisions.