If it's a normal DVD with the same capacity and structure as blank DVD's and not copy-protected like a commercial movie DVD, then there's a great freeware program that will do it called ImgBurn.

To copy a DVD with ImgBurn, you first make a copy of the DVD's stream of 0's and 1's (a disc image file) on your hard disk. Then you burn that image onto a blank DVD. That gives you a new DVD exactly like the old one.

–"What would you like to do?"
–"Create image file from disc"
–"Please select a file"
–(specify a filename and location for the DVD image)
–►
–(wait till it's done)
–"Mode > Ez-Mode Picker"
–"What would you like to do?"
–"Write image file to disc"
–"Please select a file"
–(select the file you created earlier)
–►