Disk drives on a different operating system?

Hello,
I changed my disk drive letters on one of my operating system but i think that i made a mistake. because cannot start up again, as it freezes when the welcome sign shows up, (except this time it doesnt show the welcome sign it shows the windows xp sign. I have two windows xp operating systems and I'm now on other operating system but i was wandering if i can change it on this system. Any help?