spyware
More of a threat these days than a virus but Microsoft Defender will protect you as part of your Internet Explorer. Personally, I use the excellent PC Tools Spyware Doctor it is very reasonably priced and extremely good. There is a free download, just run it and see what you have on your system as a result of Internet browsing. Most spyware is harmless but after time it will really slow your machine down. Other spyware is malicious and you really should protect yourself from it.