changed hard drive letter from (C:)to(E) by mistake

Hooked up a usb external hard drive and while following them limited instructions chemo brain here changed the (C:) drive to (E:) drive and now I can’t reboot windows xp home obviously and can’t figure out a way to get in and change it back—it didn’t recognise the new drive.Pease help me to find a solution.
Thanks
Steve