without the required contract
you basically can not install software
there are NO SECURITY updated
and it is nearly impossible to install any dependencies some manually installed or built from source program might need .
CentOS is the FREE version of the commercial RHEL
so NO you really can NOT
and I can install any software.
you NEED the support contract for that
CentOS IS FREE
there are NO support contracts required
but centos 5.7 is NOT supported
CentOS 5.11 is