I am not making this up...
aeons ago my brother-in-law's PC was behaving in a most odd manner, crashing at odd times, in a most unpredictable fashion.
His friend who 'new a lot about computers' convinced him that he had downloaded something dodgy (intentional or otherwise), hence he would need to re-install Windows and put everything back in....
Well I ran a free AV/spyware thingy, 'nowt showed up.
Nothing was obvious from log files, different programs would crash at different times, with no obvious cause/trigger.
So I opened the lid, after removing a humungous pile of dust balls that were perched over the DIMMs (can't recall exactly what was in the PC) and those by the CPU, then funnily enough it worked absolutely fine.
I do love 'amazing coincidences'!
I was quite pleased about it all...