No antivirus tool is perfect. Even the market leaders will miss a few existing threats, and some of the brand new ones, which means occasionally you might be infected by something nasty and, even if you realise there's something wrong, you'll have no obvious way to get rid of it. But that's where Runscanner comes in.
Launch the program and it'll go to work, checking all your startup and running programs, and comparing them against an online whitelist.
A "process killer" lists all the processes you're running now, and lets you terminate anything suspicious. Use the "kill and rename" process, say, and it'll close the process down, then rename its executable so it can't be launched in future.
The program also displays your Hosts file, so you can look for and delete any signs of infection. And Runscanner backs up the changes it makes so they can be undone later, if you make a mistake. (Or at least, that's the plan. If you kill and rename some vital Windows component, though, you may prevent your PC from rebooting and so fixing the problem will be more of a challenge.)
Or, if you can't figure out what the problem is, then you simply use the program to produce Hijack This-type reports on your PC, then post them online to get advice from others.
A powerful tool, but potentially dangerous if you don't know what you're doing. You'll need to be a knowledgeable Windows user to get the best from it