It's probably some sort of encryption issue. Have you used DVD43 or AnyDVD to break encryptions prior to ripping with DVDdecrypter? You could also try ripping with DVDFab decrypter which has a built in encryption breaker.
Try using the guide in my signature for DVDdecrypter and DVDShrink. Follow them step by step. Make sure you either have AnyDVD running, or DVD43 running, but not both, in the background. You will be able to tell if they are running by the icon in the task tray.
Invalid Nav Structure is usually an encryption issue, so if you are getting that repeatedly then you are not getting past the encryptions. You can also try VOBblanker. There are numerous guides for using VOBblanker here at this site, I personally have not used it as I use AnyDVD as my encryption breaker and it has never failed me.
Take you time, be patient, and report back. If you still do not get it, post your exact process steps so that we can take a look at it, and possibly your burn process log file.
i get to the bit where you have to select your files to use with dvd shrink then when i go to select the folder there are no files, although when i check the folder there is files in it, but dvd shrink must not recognise the files.
You'll need to use VobBlanker as 9mmruger1 suggested to get the job done. VobBlanker will correct the file structure porblem created by the copy protection, once corrected DVDshrink will have no problems with opening the files and peocessing them as instructed by you.
If you've already ripped the files to your HDD using DVD decrypter, you'll now need to process them thru VobBlanker, VobBlanker will create a new set of files corrected of structural problems that you'll need to store on your HDD;
NOTE: Create a folder on your desktop and give it the name of the movie your backing up...ie. Flightplan_movie..., next create another folder inside that one and name it VIDEO_TS, this is where you'll direct VobBlanker to store the files it creates in.
Run DVDshrink once VobBlanker is done and open the files created by Blanker (use open files tab), once shrink imports the files you can proceed a usual with your backup.
