Batch file to delete files with .bak extension

Be extremely careful doing global deletes! You may get more than you ask for, especially in the way of headaches and lost time restoring your system.

With that being said:

C:
cd \
del /s /q /f *.bak

For more information, type del /? from the command prompt, which gives you this in Windows 7:

Deletes one or more files.

DEL [/P] [/F] [/S] [/Q] [/A[[:]attributes]] names
ERASE [/P] [/F] [/S] [/Q] [/A[[:]attributes]] names

  names         Specifies a list of one or more files or directories.
                Wildcards may be used to delete multiple files. If a
                directory is specified, all files within the directory
                will be deleted.

  /P            Prompts for confirmation before deleting each file.
  /F            Force deleting of read-only files.
  /S            Delete specified files from all subdirectories.
  /Q            Quiet mode, do not ask if ok to delete on global wildcard
  /A            Selects files to delete based on attributes
  attributes    R  Read-only files            S  System files
                H  Hidden files               A  Files ready for archiving
                I  Not content indexed Files  L  Reparse Points
                -  Prefix meaning not

If Command Extensions are enabled DEL and ERASE change as follows:

The display semantics of the /S switch are reversed in that it shows
you only the files that are deleted, not the ones it could not find.

It can be a single command.

del /s /q /f c:\*.bak