Find a way to get to a command prompt (Bootable CD, Windows Advanced Repair)
1) Type c:\
2) Type cd users
3) If you see a list of users besides Public, then you are on the correct partition. Go to Step 6
4) If you only see the default users, then type d:\
5) Continue steps 1-4 until you see a list of usernames besides the default Windows users. Each time, change the drive letter to the next letter.
6) Once you know you are on the correct partition, navigate to
c:\windows\system32\config
assuming your windows drive is c:, or replace with whatever drive letter you figured out in steps 1-4.
7) Rename the following files adding a .OLD to the end
ren DEFAULT DEFAULT.OLD
ren SAM SAM.OLD
ren SECURITY SECURITY.OLD
ren SOFTWARE SOFTWARE.OLD
ren SYSTEM SYSTEM.OLD
8) Navigate to a folder called:
cd C:\Windows\System32\config\RegBack
Be sure to change C:\ to the drive letter you found in steps 1-4
9) Copy the same 5 files listed right above back into:
copy DEFAULT ..\DEFAULT
copy SAM ..\SAM
copy SECURITY ..\SECURITY
copy SOFTWARE ..\SOFTWARE
copy SYSTEM ..\SYSTEM
10) Reboot, then scan with something like Malwarebytes
11) If the computer will not reboot, you may need to let Windows run a Startup Repair