For windows you can get a command prompt by going to Start->Accessories->Command Prompt (I’m using XP, so this might be slightly different for yours)
This will basically drop you in to DOS (which is basically the pre-windows operating system), you’ll just need a few commands to pull off what you want.
Short answer:
Figure out what folder the files you want to rename are in and type
cd <full folder structure>
So for example
cd C:\Documents and Settings\funkdaddy\Desktop\flashfiles\
then rename them all at once with
ren *.mp4 *.flv
and you’re done
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Longer answer with explanation and references:
You can navigate to the folder you want to work with by typing “cd <folder name>” (a 20 second intro to cd) you can see a list of files by typing “dir” (intro)
Once you’re in the folder where your files are located then you’ll just need to rename the files with “rename <old name> <new name>” (intro to rename) ren or rename work the same.
You can use an asterisk (*) as a wildcard, so to rename them all at once you’d use
ren *.mp4 *.flv
And you’re done.