I want to install Ubuntu 12.04 on a Dell Inspiron 1420.
Currently that system has Windows Vista installed on it.
After installing Ubuntu, do I need to install all drivers separately?
Installing Ubuntu is usually enough, as it contains the code necessary to run on most hardware. If drivers are required, it will attempt to prompt you via an Additional Drivers
dialog.
As such, you do not need to install drivers individually.
You can also read a guide on How to Install Ubuntu 12.04.