If you shot them yourself, and they were shot with a DV camcorder, you would need a DV codec on your system to play them back. To see some availabe choices, go to www.videohelp.com and in the tools section look for "DV codecs." Athough, Power DVD seems like it has provided a codec for you now, so I would avoid installing another (it could cause conflicts). I personally used the Panasonic DV codec.
www.videohelp.com is a great place to learn about all things video, and www.doom9.org is another great resource (though in my opinion, a bit technical for beginners, but don't let that scare you!)