command line - How to rename 80.000 files at once in Windows

I've 80.000 files in a folder and I need to rename all them from

filename.jpg

to

._filename.jpg

in Windows environment, I guess from dos. The reason is that I've compressed these files into a tar.gz from unix and copied into windows and for some reason the filenames have changed.

Could you tell me what's the command to do it ?
thanks

Answer

You can use the built in rename or ren command:

ren *.jpg ._*.jpg

Though, as with all these things, try it on a directory containing just a few files first.