Attached is a script I was using for a while (drop the .txt extension when you implement it, natch). You can either create a scheduled task in the Windows Task Scheduler or you can create a SQL Agent job with the following CmdExec command:
cscript "c:\scripts\DeleteFiles.vbs" //b //nologo
Just make sure the account that the SQL Agent runs under has rights to the folders in question (the folder the script is in as well as the folder the files to delete are in).
As for the script, you can specify as many folders as you want. You can specify the maximum age of any files in those folders - anything older will be deleted (look for intMaxAge).
Note - this script will look at ALL files in the folder, not just .bak files. I had things sorted into subfolders, so I could afford to be a bit lazy with the script in that respect.
Test it first to get the parameters to what you want them to be. Be aware that the files will be deleted, not moved to the recycle bin. There is no undo.
MARCUS. Why dost thou laugh? It fits not with this hour.
TITUS. Why, I have not another tear to shed;
--Titus Andronicus, William Shakespeare