I'm new to frostwire after upgrading from limewire, and back when I had limewire, I could preview most video files with ifranview, however I never knew how I got it to do that in the first place.
Now I really need it to do it for frostwire, but I'm unsure how.

You may need to re-set the default program for video viewing on your system: Setting Default video app

Hi, Akeem...

If you are a regular user of Irfan's magic stuff you will be familiar with the way it offers you 'association' selections when you install it.

Perhaps that is all you need to do ?
Be sure to get his latest version, if you don't already have it and, in addition, maybe you need to reinstall the Irfanview Plug-ins pack... There is a newish version of them available now, also...

The above comments are 'to follow' on after taking the advice from LotR...

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IrfanView is the photo editing software. IrfanView is provided as freeware, but only for private, non-commercial use.The software is very fast and small compact.Irfanview to handle a wide variety of graphic file formats.