You'd just be stripping the DRM from the existing files. Depending on the software you use you can save to a number of formats. The current legal workaround is called Tunebites. But this is really a grey area in the law read this
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The best and legalest way to watch videos on your Windows Mobile Device is to rip your own DVDs. There are quite a few programs both free and shareware that allow you to do that and it's what the majority of users do here.