Installed openSUSE and now can't boot up except by CD or DVD

First time I have used any Linux distro and I’ve got into a mess.

I installed it on a WinXP system onto an external (USB) hard drive. I was hoping it would become a dual boot machine.

When I try to boot up with the hard drive plugged in it doesn’t even get past the Centrino logo - hangs up at that point. If I try to boot up without it plugged in I get the message
Booting from local disk
GRUB loading stage1.5
GRUB loading, please wait
Error 21

and then nothing more happens.

So I can’t boot into either windows or openSUSE. I can start KDE by booting up from the installation DVD and telling it to do an update - when it reboots after that it boots up on the external drive OK.

Go into bios and make the internal drive the first boot device then try with the USB plugged. What you did was install grub (boot loader) into the internal drives MBR replacing the Windows MBR code. So it is not seen because the machine is trying to start from the USB drive and does not find boot code. (Why it is not falling through to the next device I don’t know) .