I had the same error earlier. I searched around and found a post from a member here. I went and downloaded zlib 1.2.3 from www.zlib.net
Then did the fallowing:
>tar zxvf zlib-1.2.3.tar.gz
then read the installation instructions where you had to "delete the directory install_ispconfig, unpack ISPConfig again and start from the beginning.
I hope that helps.
PS. It would also help if you pasted the error message here.