Usually it's 'my PC was working properly, but now every time I go to the Interweb I get a popup for a porn site and my screen saver has changed to advertising an online casino and what are these six new taskbars and why doesn't my anti-virus run any more?'Plenty of times. Usually, its a BIOS setting or bad RAM stick, both easy to diagnose and fix.
The good thing about spending most of my time on Linux is that I no longer have to try to help people fix that, I can just say 'I don't know anything about Windows any more'.
BSODs are largely irrelevant, since you just reboot. The real problem is when the PC starts misbehaving because they clicked on some kind of malware install or because Windows screwed up and now Windows .Net Framework 3.7 doesn't work any more, or nothing runs because a failed Windows update just left you in SxS Hell (the latter two probably requiring a complete reinstall), or some program that should run just exits silently, maybe because something it relies on is being blocked by the Windows Firewall, or maybe because... who knows?Not really. Sure, you can make Windows run like crap by installing every piece of malware under the sun, but hey, thats true for any OS. And I've yet to see a piece of malware actually cause a BSOD in the NT 6.1 (Vista/7/8) Kernel.