Since I started learning Arabic, I have been using this program for more than 3 years and still use it, and it has helped me tremendously in increasing my vocabulary (I've rarely used any other paper or software dictionary). Actually, this program is a viewer of classical Arabic Islamic books available for download at the program's website (some free, others not), but you can download their dictionary for free (this dictionary is probably Baalbakis' "al-Mawrid", which is a dictionary of Modern Standard Arabic, not classical Arabic). It is an old program, but it works on Windows XP and Windows Vista (not sure about Windows 7 or the versions prior to Windows XP). By the way, you must enable your Windows to both read and input Arabic.
Here is the link to the program's main page: http://www.muhaddith.org/
Al-Muhaddith program download page (it's only 2.7 MB): http://www.muhaddith.org/downloads_f...are_books.html
Download page for the dictionary and its indexes (both are about 3 MB): http://www.muhaddith.org/lexicons.html
(click on Arabic-English dictionary).
NOTE: They say on their dictionary download page that downloading the indexes for the dictionary isn't important--the program will create them on its own. But I recommend that you download them anyway (it's just one file).
First, install the program. Then, copy (or move) the dictionary file and its indexes file to the folder in which you installed the program (the default name suggested by installation will be "HAD"). Then just run the program and follow the instructions.
Follow the instructions they've given. If you have any question about its installation or use, just post it in this thread.