Hi micoo, this is because another process on your server is already using port 80 (same as default agent-to-server port). This usually is caused by IIS runing on the same machine, if you're not using IIS then first disable its service and then launch again ePO install.
If you don't want/cannot disable the other software using port 80 then just change this port during ePO install (for example using port 81 instead of 80 or 800...)
I resolved this problem with
-unnstalling all the programs related with sql server express(MS SQL SERVER 2005/BACKWARD COMPATIBILY/NATIVE CLIENT/SETUP SUPPORT/ VSS WRITER)
-reboot the srever
-re-install EPO 4.5
Note : i tried to changer the port number during the installation process , but it didn't work (I still get the same message).
Thank you Laszlo G ;