Here is some tips when you works with Centos 5.5: 
- After you have installed a new machine with CentOS 5.5. Then you want to run 'yum' command to install some packages. However \
you find out that the 'yum' command did not work. The result is a error message such as: 
>>>>>
Trying other mirror.
file:///media/cdrom/repodata/repomd.xml: [Errno 5] OSError: [Errno 2] No such file or directory: '/media/cdrom/repodata/repomd.xml'
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: c5-media. Please verify its path and try again
>>>>>

the message means that the default repository to install new packages in CentOS 5.5 is from a media (e.g. DVD, CD-ROM). 

SOLUTION: 
If you want to yum to install your packages from Internet, then you have to change the system uses repository from Internet \
you open and edit the file: /etc/yum.repos.d/CentOS-Base.repo

$ vi /etc/yum.repos.d/CentOS-Base.repo

change 'enable=0' to 'enable=1' to enable the repository
and then rerun the yum command again.

Happy.