If you wanna playback files downloaded such as from youtube. You will have trouble playing back them as well as navigating as they are in .flv extension. You can convert them to other format such as .mpeg to ease your suffering playing back them later.

I found a good software that comes for free! its… Riva FLV Encoder… its very simply to use also.
Download Link

Step-by-step guide to convert.

1. Open Riva FLV Encoder 2.0

2. Click Browse at Input, search for flv you want to convert.

3. Click Browse at Output, specify the location to save the output file. Change the destination file extension from .flv to .mpeg

4. Click ENCODE! You’re done!