WINDOWS 7 problem

I have a dual boot PC with the operating systems on two partitions on two hard drives. Original arrangement was a Samsung 80gb drive equally divided. As the disk filled the partition was altered so it now shows as
Disk 2 -- SAMSUNG WIN 7 (D) 26.89gb -- SAMSUNG WIN XP OS (C) 47.63gb.
Because the Samsung disk was filling up I installed a Western Digital 160gb disk with two partitions and cloned each partition from the Samsung disk. This disk shows as
Disk 0 -- WD16 WIN 7 (G) 53.79gb -- WD16 WIN XP OS (H)
It was intended to remove the Samsung disk but every attempt to do so has failed. Without this disc connected the computer will not boot. With it connected I have the choice of booting into one of four operating systems. Although for months now I have only used the operating systems on the Western Digital disc.
With regard to Windows 7 which has problems. I get a number of error messages appearing on the desktop during boot up. When I have attempted to install programs an error message reporting that Windows Installer is malfunctioning appears. The machine seems to have altered the drive letters. They were originally on both discs "C" & "D". Now they are "G" & "H" which in itself gives problems. I would like to do a repair install of Windows 7 but there is a referebnce to Users and drive letters which I do not understand.

Can anyone suggest a course of action please.

Regard,

Wychwar