I'm always forgetting these details.
For a netinstall/network installation of centos 5.5, choose HTTP on the installation method screen. Then in the location fields input:
Web site name:
mirror.centos.org
Centos directory:
for 32bit/i386 use:
/centos/5.5/os/i386
64bit/x86_64 use:
/centos/5.5/os/x86_64