How to globally change file names

Hi,
I would be grateful if anyone can tell me how to “globally” change file
names and folder names. I have some folder names and file names contain dots(.), spaces or other charactors that I want to get rid of. The trouble is that I have hundreds and hundreds of files and folders which need name changes, and I’m hoping there is a “global” way to do this instead of having to manually change each name one at a time. The System that I use is Windows NT.

folder name example: 1a.my folder rename to: 1a_my_folder
File name example: 4a.abc ddd.doc
rename to: abc ddd.doc

Thank you very much in advance for any help you can give!

Austin