There are a lot of ways that bad actors online can compromise your computer. As their techniques become more sophisticated, it becomes more difficult to know whether or not your system has been compromised. There are some signs to look for, however, to tell if your computer is affected by malware.
You can always scan your computer with antivirus software. Microsoft includes its own antivirus utility as part of Windows 8 and above. You can also use the Malicious Software Removal Tool by pressing the Windows Key + R, typing in "MRT," and pressing enter.
Another thing to watch out for is unrecognized and unexpected popup windows. On the Mac there's a program called Little Snitch that will alert you if unapproved outbound traffic is occuring. Windows does have a firewall, but it only monitors inbound traffic. There are various other firewall products, but the problem with these is that they can be disruptive and can even keep you from getting online.
Unusual bandwidth usage can point to malware as well. If you want to dig deeper to see if there's something on your computer accessing the internet without your knowledge, there are some more sophisticated network monitoring tools to use. NMap, Wire Shark, and Microsoft SysInternals are all good options.
Watch this segment from The Tech Guy 1359 for more.