Just installed Ubuntu, but whenever I try to load from my default hard drive, grub gives me an error 18.

I have Windows XP and what I assume is where Grub is on my primary, and Ubuntu and a large NTFS partition on my secondary hard drive.

Anybody know what's wrong?

As per the grub manual, this is usually a BIOS problem.
How old is the box, and can you get a flash update ???.

Alright, I tracked down a bios update, but the flashing utility doesn't work in Linux, and I don't have access to Windows since Grub doesn't start up.

Any advice?

edit-Yea, I realize I could just re-install windows, but I don't want to spend an afternoon doing that if I don't have to...

Use your Windoze CD and boot from it in to the recovery consol and at the command prompt type fixmbr then you can boot in to windoze and flash your BOIS
Then reinstall grub to your mbr

There is a way of having your flash on a linux floppy and flashing it from there but i am not sure how to do that. I am sure some one hears knows how.

Couple of options:
- motherboard CD will often allow for BIOS update.
- some manufacturers (e.g. ASUS) allow web updates online. I don't use this unless I have the afore-mentioned CD as a backup.
- go to bootdisk.com and get a bootable floppy image. Unzip and dd the image to a floppy - copy your flash file as well, and you're all set. Need to be careful of the image downloaded - make sure it's an image, not a collection of files.

Edit: I think you misunderstood Duck2006 -[s]he was advising you simply recover the M$oft bootloader in the MBR, not re-install. That will be the simplest option. Fixing grub again afterwards is also not difficult.
Now you have a heap of options.