error loading operating system
 

I just finish setting up rh over and over and i'm getting "error loading operating system" each time. can it be because that I have /usr and /var in a different hd?

No have /usr and /var on differnt drives should not cause you any trouble. That error sounds like a bios error. Did it even load the boot loader.

shouldnt it be install be default? This is my third linuxbox that I'm setting up and I had never so much trouble with it before.

Yes it should. When you boot does it immediatly give you that error or does it start to boot and then give you that error. If it does not even start to boot than the MBR was not written correctly.

after all those boot up test and stuff.. I get a blank screen for a few second then that error message.

Make sure that the BIOS is set to boot to the correct disk. If you still cannot boot try.
Try booting to the first redhat cd. At the prompt type linux rescue. Have it mount you filesytems. At the command prompt type grub-install /dev/hd?. where /dev/hd? is the disk that you have /boot and / on (if your not sure run mount that should tell you). Then reboot.