Acabei de ver isso no ServerFault.com:
link
It's not built in to windows, but it's in the resource kit
C:>compress /?
Syntax:
COMPRESS [-R] [-D] [-S] [ -Z | -ZX ] Source Destination COMPRESS -R [-D] [-S] [ -Z | -ZX ] >Source [Destination]
Description: Compresses one or more files.
Parameter List: -R Rename compressed files.
-D Update compressed files only if out of date.
-S Suppress copyright information.
-ZX LZX compression. This is default compression.
-Z MS-ZIP compression.
Source Source file specification. Wildcards may be used.
Destination Destination file | path specification. Destination may be a directory. If >Source is multiple files and -r is not specified, Destination must be a directory.
Examples: COMPRESS temp.txt compressed.txt COMPRESS -R . COMPRESS -R *.exe *.dll >compressed_dir