No antivirus detects 100% of all threats. That’s a big issue as it means that even if you have security software installed, you may come upon viruses and other malware that is not detected properly or at all by the security software.
One of the things that I like to do from time to time is to scan my Windows PC with a standalone scanner that does not interfere with the installed antivirus solution. This basically gives me a second opinion on the state of the system.
The program that I use for this is Dr.Web CureIt, a free for non-commercial use security software that can scan the system quickly.
You first need to download the program from the developer website. The program is downloaded with a random name to prevent that the program is blocked from execution by malware that blocks the start of known antivirus solutions.
When you start the program – installation is not required – you get the option to run the program in a protected environment or normally. Click ok if you want to use the protected environment. This basically makes sure that only the virus scanner is able to manipulate the system, and that other software cannot interfere with the scanning or removal of malware.

You can click on update first to download and install the latest virus definitions. A click on start runs the system scan. The program is configured to run in express mode by default, which you can change to complete scan or custom scan in the interface.
Express can will only scan the most important system locations like the Windows system folder, all start up items, the memory and the boot sector of all disks. It is recommended to select complete scan instead to have the program scan all files on the system.

You can keep the virus scanner on your computer, to scan your system regularly with it. You do get best results when you update the scanner before you run the scan.