Ok, first things first, you need to rip the vob files to the harddrive, use a programme called smart ripper (freely available via a google search) this will "copy" the main film files then they can be played using power dvd etc but only one file at a time (a film will normally be about 5 vob files)
then visit click here
to convert the film to burn to vcd
Usual disclaimer, I assume you only want to do this for backup purposes and that you own the dvd outright.
It is illegal to copy a rented or borrowed dvd!!!!!!
Hope you have plenty of space on the hd!!