Well my original suspicion was that if when you took a screenshot it didn't appear, then it wasn't a software issue, rather a hardware issue. But in light of this new information about your admin account turning into a guest account, I'm a bit confused.
If I can remember correctly about 3-4 years a go in my script-kiddie days, I remember that with a trojan you can flip the screen horizontally or something like that. Maybe they have found one that can cause this, I'm not sure, however try degaussing your monitor and if you have any magnets near your monitor, remove them.
I thinking 'wiping' or formatting, will be the best solution to completly elimate the software factor, but I have a question, when you startup, and you have the black screen when it's loading do you see the oval thing or is it only when you login to your account?
About securing your system, well, for the past 2-3 years I have ceased to use a firewall, so I couldn't tell you the best firewall. What I can tell you is not to stand out too much in the internet community so you aren't a big target and don't accept and open everything they send you. As a matter a fact I only open .exe from people I really trust, I usually download them myself, as for e-mails it's a simple rule "Don't know u, wont open u". Paranoia isn't too bad in the internet now a days.
