Did you run TDSSkiller and deleted the \Device\Harddisk3\DR3 ( Rootkit.Win32.BackBoot.gen ) detection?
Hmm, try downloading a new version here: <a title="External link" href="http://download.bleepingcomputer.com/sUBs/ComboFix.exe" rel="external"><>Link 1</></a>
Rename it to iexplore.com before transferring it to the PC's Desktop. The click Start > Run (Alternatively, you can press the Windows key + R). In the box, type:
"%userprofile%\desktop\iexplore.com" /killall
Press Enter. See if that gets Combofix to run.