I have been running Windows 7 and 8 satisfactorily on two separate partitions on the same hard drive with both of the preview versions of W8. Yesterday I bought (online) and successfully installed the complete release version of W8 and all was fine.
Today, I decided to increase the size of the W8 partition (and decrease the W7) using MiniTool Partition Wizard. This seemed to work OK but when I enter my password for W8, all I see is a message saying "Preparing Windows" but nothing further happens. W7, in contrast, works perfectly and I can read the files in the W8 partition using Explorer.
Any help and suggestions would be very welcome.
Many thanks for your suggestion. In the end, I downloaded the setup iso to a USB flash drive and reset my PC from that. All now appears to be working correctly with W7 and W8 in tandem. I think your idea about drive letters changing may haave been correct but I had embarked on my own course of action before I read your message.