Ubuntu MBR

i have 2 hard drives, one has ubuntu, and the other has XP, after switching hard drives, neither worked, so i switched them back, and now it doesnt dual boot anymore. i installed ubuntu again on some of the extra space i have on the ubuntu hard drive, and now it dual boots again, is there a way to copy the MBR from the newly created partition onto the older one and replace it so i can dual boot without wasting the extra 2GB for the unneeded extra ubuntu? or at least where the MBR for ubuntu is? i use ubuntu 7.10 gutsy, if that helps.