Combofix is a very powerful tool, and there are other tools that can check for rootkits such as TDSSKiller.exe
and other tools.
I will ask a member of the MRT Team to assist you with this issue.
sometimes a slow PC can be the result of various factors such as not enough system resources like RAM or CPU power to handle the tasks you are wanting to perform. Other things like outdated software or drivers can also attribute to slowness of a PC. In the future I would recommend posting or asking a question about the slowness of your computer to rule out system specifications and what not then go towards analysis of your computer for any type of malware.