I mentioned two apps to backup DVDs for Mac OS X users last week. So, here's one for Windows users. I haven't tried it myself. So, please let me know how well it works for you.

DVD Shrink

Its site describes it like this: DVD Shrink is software to backup DVD discs. You can use this software in conjunction with DVD burning software of your choice, to make a backup copy of any DVD video disc.

Note that it does not burn DVD and requires some other software (it seems to prefer Nero) for that task.