Because that is where the crash comes from! come from = originate from That is why you come from China.

Rather than thinking of the word "from" as having multiple definitions, try to think of it in its original definition but just used in a different context. The word "from" means "the source of". I got a letter from Carl. Carl was the source of my letter. "I have a stomachache from the food I ate last night." The food he ate last night is the source of his stomachaches. "PCs often crash from viruses." Viruses are the source of the PCs crashes.

Because PC often crash viruses means a completely different thing. Here from means "Due to". PC often crash viruses means that Viruses are crashed by the PC which is very illogical. We mean to say that viruses crash PCs i.e. PCs crash from viruses. Also, this doesnt not sound very natural. A better way to say the same thing is, Though PCs crash a lot due to virus attacks, Macs almost never get affected by viruses.

"From" here = " as a result of". EX: The flood damage from the storm caused great damage. EX: The corruption from the virus caused my computer to crash. Though not exactly the same thing, biological viruses "give" us diseases. Thus, we "receive" the disease FROM the virus. Similar logic and usage applies to computer viruses. A "crash" of a computer is the disease/problem that is the result of an infection by a virus. The virus "Brings" a problem and "gives" it to us or the computer". Thus, we receive the problem FROM the virus. I hope this helps.

A software breakdown is called a crash. A PC (or even just a program) crashes. Viruses may cause this.