I am sending out copies of my CV, not in answer to a vacancy advert just on the off chance. I have written a covering letter but I am unsure, when i have been unable to find the name of the HR person, who I should address the letter to? Dear Sir/Madam seems ever so dated and well just wrong. It is many years since I went job hunting!
I would use google and linked in to make sure the letter arrives at the right person. That right person often isn't HR - they might run any selection process but it will be hiring managers who decide they have budget for resource. (And a speculative CV can save them any hassle of going to HR!).
You will have a much better chance at getting over the door if you target each letter. Takes more time but more likely to yield results.
Why do you want to work at THAT company? What are they well known for? What do you have to offer THEM?