If you boot on BartPE and load registry hive, you will find many "paths" looks for the old letter so you will have many errors ... To pass logon, you can change (always from BartPE and with registry hive loaded) HKLM\Software\Microsoft\Windows NT\Winlogon\ userinit value to YourLetter:\Windows\system32\userinit.exe, By the way, I have had many systems that have C system partition "will become" (sorry for language fault ..) E or D for example. No really problems after (it seems its Windows that's changed letter, with upgrading ou repair install perhaps ... ) but unable to properly re-set to C (even with many registry ...