Why ? Yum and rpm are always installed by default.
And : All the links you show are to the non existing CentOS 5.7 .
The current version is 5.8
http://mirror.centos.org/centos/5.8/
x86_64
http://mirror.centos.org/centos/5.8/os/x86_64/CentOS/
Updates
http://mirror.centos.org/centos/5.8/...s/x86_64/RPMS/
Yum dependencies :
python, rpm, yum-fastestmirror, python-urlgrabber, yum-metadata-parser,
python-elementtree, python-sqlite, python-iniparse, rpm-python.
.