|What profit is there in most computer viruses that don't steal data? Very little.|
Plenty, actually. They're interested in using your PC's processing power and using your Internet bandwidth (mostly the bandwidth). Any time you read about a DDoS attack, it was powered by a botnet. Sending spam is also a common task. Basically, whenever you want to anonymize your less-than-legal activities, you use a botnet. I've also heard of botnets being used to store "things" for clients, but I haven't heard of that for a few years. Either the owners of the infected PCs are just that clueless, the unreliable nature of such storage makes it undesirable, or the black markets fueling these botnets frown on it. Low to no visibility is obviously the goal of these viruses.
There's also extortware, those viruses posing as anti-mallware programs, that charge you to use your computer. Low to no visibility after you pay is the goal of these viruses.
I agree, the issue must be in the boot sector but I have no idea how I'd fix it.
Going by your description, it sounds like it's happening before the BIOS runs the boot loader.
But that doesn't change the 'other user' screen being a known piece of malware.
Actually, without more information, it sounds like you're looking at the Fast User Switching screen. It's a built-in but seldom used feature.
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