If you want to install on the system as it is, just install the new Linux distro. Have the installer delete all Linux partitions, and make new ones. Have it install the bootloader, just as you did for RedHat. It will take over, as if RH was never there.
If you remove the first hard-drive, as it is, the system won't boot, because the Master Boot Record points to the /boot directory, which is prob'ly on your second drive. You can restore the MBR to point to the first partition on the drive and boot straight to Windows. If you have 2000/XP, it will be in the disk control. If you have 95/98, then you will need a boot floppy, and run "fdisk /mbr".
Of course, restoring the MBR will stop your Linux install from booting. Since you are going to re-install, it won't matter. But, if you are going to pull the drive out, you should restore the MBR on it.