I have two serial ATA 80GB drives that I originally configured in RAID0. Then I installed XP Pro on the first 60GB and an extra 15GB NTFS partition. Later, I decided to install Ubuntu 7.04 as well. When I went through the installation, I selected to use the entire sdb partition. After the error 21 message, I tried to load the XP startup disk to recover the MBR, but it would not load. Anyway, from the Ubuntu live CD, I ran fdisk -l:

Disk /dev/sda: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/sda1 * 1 7833 62918541 7 HPFS/NTFS
/dev/sda2 7834 9138 10482412+ 7 HPFS/NTFS

Disk /dev/sdb: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/sdb1 * 1 9541 76638051 83 Linux
/dev/sdb2 9542 9729 1510110 5 Extended
/dev/sdb5 9542 9729 1510078+ 82 Linux swap / Solaris

Also, here is my menu.lst:

title Ubuntu, kernel 2.6.20-15-generic
root (hd1,0)
kernel /boot/vmlinuz-2.6.20-15-generic root=UUID=5ea6fecf-55ee-4421-a1dc-05e7ca2d1f0b ro quiet splash
initrd /boot/initrd.img-2.6.20-15-generic
quiet
savedefault

title Ubuntu, kernel 2.6.20-15-generic (recovery mode)
root (hd1,0)
kernel /boot/vmlinuz-2.6.20-15-generic root=UUID=5ea6fecf-55ee-4421-a1dc-05e7ca2d1f0b ro single
initrd /boot/initrd.img-2.6.20-15-generic

title Ubuntu, memtest86+
root (hd1,0)
kernel /boot/memtest86+.bin
quiet

So, I'm not exactly sure what to do now. Thanks in advance for your help.

Do I understand you that you installed Ubuntu to RAID?
But looking at the data you provided, it looks like you have no RAID now. Possibly you had removed the RAID uncleanly?

That XP startup disk would not load is very odd... unless it is the kind to require a rescue partition. There is a possible hardware failure here.

Error 21 is unhelpful... it just means "unknown boot error". It does mean tha GRUB got to stage 2.

All I know is that I set up my XP install under RAID0. When I went to install Ubuntu, I chose the gudied partition, and then selected setup on sdb.

By the XP disk, I meant the original installation disk. It starts to boot from the CD, but then stops loading after a few seconds.

I am going to try and reinstall Ubuntu with the manual partitioning to see if that helps anything. I am guessing that the main problem is caused by the RAID setup.

Yep - it sounds like the Ubuntu install has buntad your RAID.
Reinstall everything.
Do not use RAID.
Install windows to the first sata drive using windows installer.
Install ubuntu to the second sata drive with the ubuntu installer.
Do not use anything else.
Put grub in sda0 (first sata drive MBR).
This way you double your HDD space over the RAID0 (see below) and you won't notice the speed drop as your RAID drives were on the same controller. If you have HW RAID in BIOS, you'll probably be best to disable it.

Note: RAID0 is striped
http://www.acnc.com/raid.html
... when you installed Ubuntu to the second RAID physical drive, you lost all your windows data. The only advantage of this setup is data r/w speed.