Hard Drive listed as E drive

I have an XP computer that was set up as a fresh install. After installing the OS, I attached my old 98 hard drive to migrate data over. After the migration, I disconnected the 98 drive but what should have been my “C” drive was showing up as the “E” drive. I tried to change the letter in disk manager but since that’s where the system (or boot) partition is, it won’t let me change it. How can this be changed to the “C” drive?