If your PC has been infected by malware then you might find this prevents you from regular antivirus software, making it very difficult to get clean. But that's where RKill comes in, because the program will try to kill those malicious processes, as well as carrying out some basic system repairs, so that your security software can remove all the threats.
It is important to understand that RKill will only terminate currently running processes, though. It won't delete the original files, and if you reboot then the malware will launch again. So the program isn't a substitute for a real antivirus tool, it's just a potentially very useful accessory which might help if your standard security software has been compromised.
If that sounds interesting, then the good news is that RKill is extremely easy to use. It has no options, no configuration settings, no interface, really - just launch the executable, it'll go to work on your PC, and a console window will display exactly what it's done (a plain text file report is also saved).
You do still have to be very careful about exactly how and when you use RKill, though. When we tried it on a test PC, for instance, it terminated four processes which weren't malicious at all. This is most unlikely to cause any critical problems (reboot and they'll be back), but there are no guarantees. So we would only run RKill when you're 100% sure that your PC is infected and regular security software won't be able to help. And even then, you should save all your work and close any open applications before you launch it.
As long as you keep this in mind, though, RKill could prove a very useful tool of last resort. Just keep in mind that some malware may also know about the program, though, and try to prevent it from running. That's why the Download button here links to the official RKill page, where they have multiple file links with a range of apparently innocent names (iexplore.exe, userinit.exe, winlogon.exe, explorer.exe and so on): try each of these until you find one which the malware will allow you to download and run.
It's very much a tool of last resort, but if your PC has been infected by malware which is stopping you running security software then RKill may be able to help