Unless you are specifically told not to, you should always write a covering letter to accompany your CV. This will help to ensure the CV gets to the right person, and will also give you another opportunity to advertise yourself.
Use the covering letter to introduce yourself, say why you are interested in the position, where you heard about it and also say why you feel you are the right person for the job. However, be careful not to simply repeat what it says in the CV. Use the job description as a guide, but don't address every minor point in turn or the letter will sound very boring.
State when you are free for interview, or would be able to start work were your application successful. If the job is advertised as having a negotiable salary, do not mention this here. This is something you can discuss much later – when they offer you the job.
The covering letter is not an opportunity to go on and on, however. Make sure you cover the important points, and leave it at that. Never make your letter any longer than a single page.
