DVDFab hasn’t been called Platinum for a very long time now. You must have an old copy.
You can update DVDFab here: www.dvdfab.com
You may not have the option to convert dvd-video (which is what I assume you mean by VTS files) into avi unless you pay for that part of DVDFab. There is a free section of DVDFab that will work as a decryption/ripping program to extract the movie from the disk and rip to the hard drive. It is called DVDFab HD Decrypter.
There are other, better options that don’t cost anything for avi conversions. Once you have the dvd decrypted and on the hard drive, you can use a program like AutoGK or AviDemux to convert them.