For all versions of Windows, pressing the F8 key at boot-up is the standard method of bringing up a startup menu where you can choose what mode to boot into. The menu options differ between different versions of Windows, but Safe Mode is always one of the options.
In safe mode, Windows starts/loads only the most basic components that it needs to function. Many malicious programs are programmed to start automatically when Windows is booted in normal mode, so by booting into safe mode you keep these programs from loading, and thereby keep them from interfering with your spyware and virus removal efforts.
Does that answer your question?
(By the way- yes, Kazaa and other filesharing programs are some of the worst offenders when it comes to distributing adware and spyware.)