You don't have PHP DOM installed. This can be done either at php compile time or after, depending on wich distro you're using.
For centos5, you shoud do:
yum install php-xml
For CentOS 4, if you installed php from centosplus repo (that's php-5.1.6), you should do:
yum install php-xml --enablerepo=centosplus
For other distros or for windows, I've no idea. Hope this helps. Just keep in mind to install php-xml or php-domxml, depending on your distro.
Best regards from Romania