I had this problem with Windows 7 yesterday, so I thought I'd add how I fixed it. It started when I removed a partition, causing the drive letter of my Windows partition to change, meaning it couldn't boot. I fixed this by generalizing the BCD so it is based off wherever it boots from (see Multibooters, Vista Dual and Multibooting - Cloning Vista
). Now it booted to the login screen, but the drive letters were still wrong, so profiles that explicitly state the drive letter (such as mine which are on another partition) didn't fully load (hang at Welcome screen).. Fix this by changing the MountedDevices key in the SYSTEM registry (How to restore the system/boot drive letter in Windows
and Multi-Booting with Windows in an Extended Partition
for detailed info). If you have to do that via Recovery Mode or a Recovery CD, go into regedit in Recovery Mode and add the hive from your installation (in \Windows\system32\config\SYSTEM) and edit it via there.
The point of this post is not only did having the wrong drive letters disallow logging on (directly and via Remote Desktop), it also disabled the mouse and keyboard at the login screen. I have no idea why the drivers
need correct letters when everything else seemed ok, but that's how it is.