This problem has nagged at me for years. Here is a batch command to delete files on a Windows 2003 machine.
Forfiles -pC:\backup -s -m*.* -d-5 -c "cmd /c del /q @path"
This will delete all files in my backup directory older than 5 days. To test it first, use this:
Forfiles -pC:\backup -s -m*.* -d-5 -c "cmd /C Echo 0x22@Path\@File0x22"
This will print out each file that you will be deleting.