The compression package "zlib" is part of the zip directory. The original tar 
file can be downloaded from : http://www.zlib.org 

Only the useful code has been extracted from this tar file. It consists of: 

 zip/inc:
   crc32.h deflate.h inffast.h inffixed.h inflate.h
   inftrees.h trees.h zconf.h zlib.h zutil.h

 zip/src:
   adler32.c compress.c crc32.c deflate.c gzio.c infback.c
   inffast.c inflate.c inftrees.c trees.c uncompr.c zutil.c

To put a new zlib version in ROOT, all these files must be replaced.

The zlib version number can be found in zip/inc/zlib.h

zlib is now used in TPDF to compress the "stream" part of PDF files.

The ROOT compress functions R__zip and R__unzip have also been modified
in order to use zlib. Some performance tests (on a P4 2.4GHz) show that 
the new zlib version is as fast as the current ROOT zip functions when
writing and up to 10 per cent faster when reading.
                          oldzlib            newzlib
Event 1000 1 0   1         11.59s             11.46s
Event 1000 1 0  20          3.83s              3.31s
Event 1000 1 99  1         10.44s             10.98s
Event 1000 1 99 20          3.57s              3.17s
stress -b 30               13.01s             12.35s
stress -b 1000             61.67s             58.38s
bench                     151.99s            151.56s
stressLinear               35.87s             35.77s

To activate the new zlib it is enough to set Root.ZipMode = 1
in the system.rootrc file.
When setting Root.ZipMode to 1 (or 0) it is possible to read files
written with the old zip function.
Files written with Root.ZipMode=1 can only be read by versions
of Root >= 4.02/00