Look at the date of the backed up registry files in C:\Windows\System32\config\RegBack. They should be dated prior to this problem occurring. If so you can save the current registry files and replace them with the backup files.
Boot from install DVD or flash drive, take the Repair my computer link to get to a Command Prompt. Do the following from the command prompt:
Change to the directory the registry files are in:
cd C:\Windows\System32\config
Prompt should be C:\Windows\System32\config>
Create a new directory to save the registry file in
md saveold
dir saveold should show the directory exists.
Copy the current registry files to the save directory:
copy *.* saveold\
dir saveold should show the files in that directory now.
Now copy the backup registry file over the existing files in the current config directory:
copy regback\*.* . (note the trailing . )
Reboot normally, see if the problem is gone. If not or if problems, repeat the above but restore the registry files from the saveold directory to the config directory.