When a virus got hold of the start-up (rebooting) process and you can not even get into the desktop,
you can still stealthly get the control back by a back door using Task Manager.
Even if you may not get complete re-boot of the Windows, yet the "Task Manager" can be still functional.
This is the secret of this maneuver. It does bypass the viral control of your computer and go behind
and execute the "rkill.com" to kill the virus in the memory and then run antivirus program
without getting into windows re-booting.

1. On the virus screen, hit Control-Alt-Delete to invoke Task Manager.
2. Among the applications, if you can "end Task" of the viral process, do it by all means.
3. Most of the time, chances are the virus will keep you from doing that.  
4. On the Task Manager Window, at the right lower corner, hit the "New Task" button.
5. That will open a "Create New Task" window. Now, you are going to create the "antivirus" task.
6. Hit the "Browse..." button at the right bottom.
(You have the "Rkill.com" and Antivirus program in your computer.
Always keep them at the desktop or under C:\ to get to faster.)
7. Open the "rkill.com" by finding it by browsing.
8. Then go ahead execute by clicking "OK".
9. After rkill is done, browse again for your antivirus program.
Once it is on the Open input area, go ahead and execute and scan the computer.
10. It will find the offending virus or two while doing that.
11. Remove the virus by antivirus program.
12. Then, try to reboot.
13. If rebooting is successful, run your antivirus for a complete scan again.

There may be another back door approach to execute rkill and antivirus.
This is by invoking the CMD.exe in the basic (non-regular) bootup.
At the MS-DOS cursor, type "dir C:\..." to locate the rkill.com and antivirus.exe
Once you know the exact locaction, execute by typing the directory tree to the very file.

Make sure you do not keep these programs in C:\Program Files or C:\Program Files (x86),
because DOS command can not contain "space" and therefore it will never find
this two directories.
Keep the antivirus program and rkill either "C:\" or on the desktop.
Then you can get it by DOS cmd.exe.