Possibly you know other ways to do it, but I didn't know how so I had to figure by myself. I'm sharing the tip in case it's useful for someone.
I had a directory with 1200+ files and wanted to add a random number before to the filename. The file renamer I use, and several others I've checked allow sequential, but not random, numbering.
I finally came across a freeware app: Webmastersguide Renamer, http://www.webmastersguide.com/downloads/renamer.exe that allows to add a random number at the end of the filename, but not at the begining.
Later I found another freeware app, Siren http://perso.wanadoo.fr/rplr/scarabe.../Siren_170.zip, that allows to change the order of parts of filenames (amongst many other things).
With these two toools it's easy:
1 - Rename your files adding a blank, a hyphen, a blank and then the random number with Webmastersguide Renamer
2 - Change the order of the filename, putting first the number, with Siren (check the examples the program has to learn the expression you have to use
Et voila