Cannot connect to X11 window server using ":).0" as DISPLAY variable
I'm totally new to Linux and only just installed RedHat Enterprise 4 on a machine today. I've got the basics set and i'm now trying to install Oracle 10g. I've been following the installation guide from the Oracle website -extracted the Oracle files from the download and set up the Oracle users etc but when i try and run the installer (logged on as the Oracle user) i get the message 'Can't connect to X11 windows server using ':0.0' as DISPLAY variable. I've tried changing the DISPLAY to localhost, localhost:0.0, ipaddress:0.0 and anything else i can think of but get the same error each time.
I've tried running xhost +localhost under the root user (which most searches seem to bring up as a solution) but it doesn't seem to help.
Any ideas would be great.