This seems to be appearing a lot at the moment..I think it may be spyware or virus related but it is a real headache to resolve...
If the following doesn't work then you are probably looking at a reinstalling windows...
Click Start, and then click Run.
In the Open box, type cmd, and then click OK.
At the command prompt, type the following commands, pressing ENTER after each command:
PLEASE NOTE : THERE IS A SPACE BETWEEN THE TWO SECTIONS THIS IS CRITICAL
expand c:\windows\i386\config.nt_ (space here)c:\windows\system32\config.nt
expand c:\windows\i386\autoexec.nt_ (space here)c:\windows\system32\autoexec.nt
expand c:\windows\i386\command.co_ (space here)c:\windows\system32\command.com