As described above, regular scanning with online tools is a good idea. Consider also trying an offline scanner, such as the one AVG provides free of charge. This requires downloading an .ISO file containing a standalone Linux variant, the scanner and related files. You burn the .ISO file to a CD, which you then use to boot the computer and run the scanner.
This allows you to "look aside" at your PC's files without running any of them. This foils possible malware attempts to suppress online scanners. I have used this technique to rescue PCs from nasty, escalating ransomware that was untouchable online.
You might also create a DVD from which you can boot & run a full-blown Linux system without actually installing it on your PC. The no-cost Ubuntu Linux installation download provides this capability, for example. You can use this to backup your data files before attempting any anti-malware cleanup. Data backup might require hours, but this is cheap insurance.
Note that malware "infects" only executable files (.exe), code libraries (.dll) and boot-related files. However, malware can create new data files and/or add information to otherwise innocuous data files. If such modified data files remain on your PC after you have otherwise cleaned it up, a subsequent infestation of the same malware might be able to read and use them.
Also, certain data files blur the distinction between data and executable code. The scripting capability available for inclusion in MS-Office data files provides a notorious infestation vector, for example. This is why MS-Office applications now disable scripting by default. Always treat with scepticism any instruction to allow running scripts baked into MS-Office data files you may have downloaded.
I run Linux as my primary OS. Linux is inherently less vulnerable than Windows. It also presents the bad guys with a much smaller, and frankly more knowledgeable, target population. I run MS-Windows inside a virtual machine on Linux, mostly to use Coffee Cup's products and to view my Web work in genuine MS-Internet Explorer. You might consider doing the same thing.
1) Keep your Windows image up to date with the latest updates.
2) Check your firewall and close any unnecessary entry points.
3) Scan regularly with one or more online tools.
4) Prepare and learn how to operate an offline scanning / repair tool and offline backup tool BEFORE you have a problem. Once you have a problem, you can't 100% trust the integrity of anything you attempt to download into your PC, since already resident malware could theoretically modify it.
5) Avoid running scripts. Take the time to understand a script before you run it.
6) Look into migrating away from Windows. This is lots easier now than it was a decade ago.
halfnium -AT- alum.mit.edu
dba New England Radius
Yes, I looked just like that in 1962.