Ok firstly this is how I do it on Moodle 1.9+?... yes I know I need to upgrade
On the front page of your moodle -> Admin Block -> Reports ->Statistics then chose your course form the drop down menu. Have a play with the report type and time span until you get teh sort of data you are looking for, I tend to use a maximum of 6 months data and after that for some reason on my Moodle it starts to look a bit Blurg
Hope this helps