Welcome to the CentOS fora. Please see the recommended reading for new users linked in my signature.
Depending on the source hardware, and how good a job the converter does, you may find a need to read http://wiki.centos.org/TipsAndTricks/CreateNewInitrd
You should also be aware that CentOS-5.7 is the current release. See the CentOS 5.7 Release Notes
for details. By not updating you are implicitly accepting that you will live with numerous bugs and security issues (and associated known exploits) that have subsequently been fixed.