Question : Problem: Dell  23 inch Monitor not displaying full screen on Vista

A collegue has just purchased a new Dell computer with 24" screen

It arrived with XP installed and he updated to Vista but now on monitor a black border of about an inch goes around the screen.

There must be a setting in Vista that is not done .... Any ideas?

Answer : Problem: Dell  23 inch Monitor not displaying full screen on Vista

Try go to "control panel"  - "Appearance and Personalization" -> adjust screen resolution.
Change the resolution to 1900x1200(Native to 24 inch monitors)

If you can't change it or it doesn't have the 1900x1200 option, I'd suggest check if correct driver is installed for the graphic card and the monitor.
You should be able to get the specification of the graphic card from the mammal or online, download the driver(Vista Driver) for it - install/update the driver, you should be able to set the monitor to use the right resolution.