Updating your computer is almost complete virus
This question comes up frequently, and the suggested solutions are usually the same.
This community wiki is an attempt to serve as the definitive, most comprehensive answer possible. One thing to definitely NOT do is to install any of the "anti-malware" tools that you're urged to when you get to a web page that says "Your computer is infected by a virus! You must only use tools that are well-vetted -- (presumably) those named below or on another trusted site.
The best place to start is visiting our page on regaining hard drive space.
Malware can hide in your files, your application programs, your operating systems, firmware...
The more files you have, and the slower your computer is, the greater the time the scan will take.