GRUB Error 21 problem.

Hi, all. I need help from anyone here who knows my problem well. I am a veryt new linux user. Presently, I have installed ubuntu successfully in a USB harddisk, and my existing hardddisk is in Win XP which I didn't changed anything to its partition. When I reboot PC, I can see the GRYB boot loader in action providing the bootup option configuration menu: Ubuntu or WinXP. I can boot either one when I select either one. Now my problem is, when I remove my USB harddisk and reboot my PC, the GRUB boot loader gives an error msg "Error 21". The PC hangs there and my Win Xp won't boot up. It seems that the boot loader has infiltrate into my win XP boot sector although I didn't do anything to winXP harddisk during ubuntu installation. As Clayton mentioned above on similar issue, I didn't change the win Xp harddisk jumper to slave, so maybe that's the problem. I reckon now that even I reformat my ubuntu USB harddisk, it won't help as my win XP boot sector still has GRUB boot loader stuffs. But now, without reformatting both my Win XP and Ubuntu to restart all over again, I wonder if there is any existing and quick solution to make it such that even after I remove my USB harddisk, my win Xp harddisk can still bootup.

Need desperate help.

Ken Teo