Emsisoft HiJackFree is a powerful tool for investigating your PC, helping you detect and remove any malware which might have bypassed your regular antivirus tool.
This might start by displaying all the processes and services running on your system, for instance. Windows can do something similar, but HiJackFree goes further, filtering out operating system and other trusted files, helping you zoom in on possible threats.
The program can show you which processes currently have an open network connection, for instance. Or you might use the Autoruns section to browse your Windows startup programs, scheduled tasks, and other commonly exploited areas.
There are plenty of bonus tools here. You're able to check your system HOSTS file, any Explorer shell extensions, LSP addons, ActiveX controls and more.
And if you do find something suspect, then it can be quickly moved into quarantine. This ensures the object won't be able to do any more damage, but also means you can easily restore it later, if it proves to be harmless.
A useful collection of tools which could help expert Windows users to manually detect and remove malware