You can do this easily with a batch file as follows:

From the Command Prompt (available under accessories) run the following from the directory containing the files:

ren Branch????_2010.* Branch????_2011.*

You change directories with the command cd DIRNAME where DIRNAME is the directory name. This can get a bit weird if you are not used to batch commands, so alternately, you can place a text file in the directory which contains the above, rename it to RENAME.BAT, and then double-click it to execute the command.

---GJCase