Do you have another computer on the same broadcast domain as the target computer, and do you have low-enough level access to the machine to be able to change BIOS settings to let it boot via the Ethernet card(s)? Otherwise, you're not going to be able to do much.
If the remote access you have is IPMI/DRAC/some other integrated remote access card, these will often (always?) provide the ability to mount an .ISO as a drive and export it to the target computer. You may wish to consider this approach.
As far as a way to PXE boot CentOS 5.5, we use Cobbler to do this all the time; it's a simple "yum install cobbler" and then take it from there. However, it's hardly plug-and-play, and there's likely to be a simpler approach if you only need to do this for one machine.
One last postscript: why 5.5 and not 5.6?