"Hi, I have a Dell SP2309W 23-inch Full HD comptuer monitor with HDMI input. Can I connect my high definition set top cable box HDMI output to my computer monitor directly to use the monitor as an HDTV?"
The differences between an HDTV and a computer monitor of the same size have become minimal. If you have a computer monitor with a screen resolution of 1920x1080 and HDMI input, you effectively have an HD-ready television screen. Conversely, if you have an HDTV with an easy way to connect to your computer, your HDTV can make a great computer monitor. In your case, all you should need is an HDMI cable to connect your cable box to your computer monitor, however, there are a few cases where I have seen computer monitors fail as an HDTV screen.
The primary instance where a computer monitor will fail to function as an HDTV screen is when the monitor manufacturer failed to include support for the HDCP content protection protocol. For many types of HD signals, including Blu-ray and some HD cable channels, HDCP is used to protect the content from copying. All the components involved in displaying the HD content need to support HDCP in order for the signal to work. Dell, HP, Lenovo, Samsung, and other name brand monitors all include HDCP support. I have seen cases where off-brand computer monitors don't include HDCP, which could make the connection between the cable box and the monitor fail.