Take a look here: ComboFix usage, Questions, Help? - Look here
From the above link: I used ComboFix on a 64-bit system and cannot get it to work?System Requirements
: Combofix currently only works with Windows 2000/XP/Vista/Windows 7 (32-bit
Due to the architecture in 64-bit windows, drivers need to be digitally signed. Windows 64-bit enforces driver signing so rootkits cannot reside on that system unless someone is going to issue certificates to malware writers. Since drivers need to be specific, 32-bit drivers do not run on a 64-bit operating system. As such, rootkits are not seen as often on 64-bit machines so they are less prone to that type of infection but with technology that may change. Right now, the tool's creator has indicated it is very unlikely that there will be a 64-bit version of ComboFix since that OS is more secure than a 32-bit system. Note
: Although ComboFix will work on Windows 7, it is not
officially supported yet so if it is run you will receive a warning message
that it is a beta version meant for compatibility testing.
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