You're asking if you can use a computer monitor as a TV?
This really depends on the connectors present on your cable box/dvd player and monitor.
Converting from Composite to VGA, DVI or HDMI doesn't usually work very well.
Converting from Component to VGA, DVI or HDMI doesn't usually work very well either.
An HDMI output from your device to an HDMI input on your monitor would work.
Composite:
Component:
VGA:
DVI: