There are a number of programs that will rename a load of files for you.
I have never used one so cant say what one to use.
Search under "batch rename program" or "bulk rename program"
Whatever one you do use, make sure you take a copy of all the photos before playing around with them in case it all goes wrong.
Here is one program I found, no idea if it is any good (there are loads of other program which do the same thing).
http://www.bulkrename....co.uk/Main_Intro.php