Hi,
I had Error 18 on Grub.
Changing the HD from Single to CS mode did the trick for me.
My configuration : PIII 750, DD 40GB, Ubuntu 7.10.

I've been reading some of the answers to the same problems I'm having.
Grub loading, please wait...
Error 18

I tried a few different suggestions that seamed logical to me.
But it didn't work.

I originally installed UBUNTU on a primary HD with a secondary HD that had Windows XP installed on that.

It booted up but,
I couldn't do anything with it...

It said that I couldn't copy over to any folders cause I had no rights to do that.
WTF?

So I took off the secondary HD and I can't stop getting these problems.
To me this program sucks.
I'm going to look elsewhere.
Good luck to you all...

Instead of placing grub into the MBR (which is the default for Debian 5 Lenny, and has been suggested above as a work-around to the error 18 message) I had to create a small (200 MB) /boot partition at the beginning of the disk and place grub into the partition's boot sector - in my case into (hd0). Interestingly, the motherboard was brand new - a Gigabyte GA-MA78GM-S2H, and yet grub couldn't handle the 360 GB disk.