Safemode
Safemode, maybe you saw the option listed in the windows bootmenu, maybe you heard people talking about it or maybe you know exactly what it is.
In safemode Windows will appear quite basic, no fancy graphics, no hardware you can use and no itnernet connection, unless you start safemode with networking support (which I strongly disapprove because when you're infected the virus can spread itself or your computer can be at risk of even bigger problems). This 'basic' windows loads is generated because only the essential/core components of windows are loaded, no hardware drivers will be loaded, no programs which normally start at boot and, that's where we come in, no viruses adware or spyware is loaded.
When no virus spyware or adware is loaded the programs can't 'protect' themselves. Some viruses, as wel as other malicious software, protect themselves from deletion when booting windows normally. Since these programs are not loaded in safemode it's much easier to get rid of them.