File renaming question

I need to add four characters into the middle of numerous filenames.  I have tried Batch Rename in Bridge CS3 but did not have any luck. Tried macros with photoshop, and also can't do it.  The file names would look something like this:  11-100.jpg  and I need it to be 11_VX1-100.jpg. Then the next file name would be maybe 20z-100.jpg and I would need it to be 20z_VX1-100.jpg.  Someone suggest using scripts. I do not know how to use scripts. Can it be done? and How do I do it?  Thanks!