The authorized site for downloading ComboFix can be found in the ComboFix usage, Questions, Help? - Look here
topio. The site is specifically noted in red at the bottom of that page with a WARNING not to download ComboFix from anywhere else. The actual download link is provided in the authorized guide: How to use ComboFix
...you should download ComboFix from one of the following URLs:To download ComboFix, click the link above, and at the page that opens, please click on the download link for ComboFix.
If you are dealing with a malware infection, please be aware that using ComboFix is only one part of the disinfection process. Preliminary scans from other tools like DDS
should be used first because they provide comprehensive logs with specific details about files, folders and registry keys which may have been modified by malware infection. Analysis of those logs allows planning an strategy for effective disinfection and a determination if using ComboFix is necessary. ComboFix was never meant to be used
as a general purpose malware scanner like SuperAntispyware or Malwarebytes' Anti-Malware which scan individual drives or different folders on a computer for viruses.