ComboFix is mostly a collection of batch files that uses some command line tools, some of them from NirSoft. Sunbelt security software has been, for some time, detecting some Nirsoft tools as Backdoor.Bifrost and should know better by now.
You can see the kind of tools NirSoft offers here:http://www.nirsoft.net/
There are a number of small utilities that get flagged by various security software, usually called something like "riskware", "potentially unwanted software" and various other designations--such tools can be used for good or ill--they are warnings and so I call them near-positives because they aren't mistaken for actual malware. That is why SmitFruadFix came into this--it uses such tools as well. But Backdoor.Bifrost is an outright false positive and Sunbelt should correct that.
I would actually suggest you not use ComboFix at all. If you fix computers for a living, you should know how to do more than run automatic removal tools to fix infections, so using tools like NirSoft's would come easy to you. There is a reason ComboFix isn't offered to the public--you won't find it at download.com or any other download site. The author of ComboFix did not design it for people who are infected to use. It was designed for the people who help the people who are infected and them only, in forums like this one:http://www.bleepingcomputer.com/forums/f/22/virus-trojan-spyware-and-malware-removal-logs/
The author supports his tool in private forums for malware removal helpers. So you would have to be a malware removal helper that volunteers their time and help for free to be able to get support for CF. That is why disclaimers are plastered all over the internet that CF should not be used without supervison of a malware removal specialist. If something goes wrong, the specialist can get support from the author to resolve any issues. If CF is used unsupervised, you are on your own and could end up with a funny looking doorstop.