Lingoes is a free dictionary and full text machine translation software. Just click on the image above, if you like to find out more.
I have been using it for one year (the dictionary, not the translation software) and am happy with it, makes my work a lot easier.
You can download dictionaries according to your needs. The dictionaries may not be the latest copy but it is good enough if you need a quick check. Use it at your own discretion 
So far I have not encountered any security or virus issue with Lingoes.