Drive letters won't appear in some boot disks because they are different in the boot environment than in the OS.
What repair did you think you were going to run that involves changing drive letters. You cannot change an OS drive letter without ruining the OS.
The steps are here for Troubleshooting Windows 7 Failure to Boot.
If it's not to late here are the best practices for Clean Reinstall - Factory OEM Windows 7
which are same for retail.