I'm looking for some general sage guidance/advice from people who know this field.

I'm not entirely certain that this is possible to answer in a single post, but I'll give it a shot.

First, some basic advice:

• If you have no clue what something is, learn as much as you can about it and then make an informed decision.
• Before you do anything that you aren't 100% certain isn't going to break a machine, make a backup. Hell, make a backup anyway.
• Just about anything that isn't hardware-related is fixable, but that doesn't mean it's worth the time. If you can recover data, and think the problem is going to take more than a few hours to fix, it may be better off starting from scratch.
• Occam's Razor. The simplest solution is more likely to be the correct one. Computer won't turn on? Your first step should be to check if it's plugged in, not assume the power supply is bad.

Now on to some of your other questions. In regards to viruses and malware, the best way to handle them is two-fold. Firstly, don't be stupid. If you've never heard of a program, and can't find more information on it, don't install it. Secondly, a good anti-virus program will help act as a safety net if you do in fact accidentally install something. Generally speaking people widely consider nortan, mcafee, and a few others as pretty terrible at doing their job. Some of the one's that actually perform well are eset's nod32 and kaspersky. There's plenty of research online on the strengths/weaknesses of each.

Generally speaking, the key to keeping a computer running smoothly is keeping up with updates. You can improve performance by clearing things like temp files, caches, dead registry entries, etc. but nothing (at least for windows) will beat a good clean start every 8-12 months. Obviously, that's unrealistic for most people, but thankfully windows 8 includes a decent refresh os program that seems to do a good job.

It's hard to give an overview of something that is pretty much entirely experience based. The best I can tell you is to tackle each problem as it comes, be careful in what you do, and be prepared to trash many an OS install. Eventually, you'll learn what is okay and what isn't.

Finally, lurk on this subreddit. Read everything, see how people attempt to solve it, see what works and what doesn't. Pay specific attention to the resolved posts. You need experience more than anything.

If you have a specific question, feel free to send me a PM.

Basically, I want to know what my biggest vulnerabilities

That's easy. BKAC. Between the Keyboard and the Chair. This isn't meant to be disparaging, but your first line of vulnerability is always yourself.

You are the one who is using the computer. You do things, and they can be trouble.

Your second line is what the people who made the parts that are on your computer and the related hardware have done. Things like Flash, Java, can be vulnerable.

Where to start beyond that, that's a bit hard to address. You could try to find some basic computer books, and transition to more advanced ones, or you could try things like Linux How-to's that are more generic, or RFCs, but I don't know of any good process to learn through.

This is a big topic to cover, so I'll just give one piece of advice about software installation:

When choosing to install a new program, how difficult was it to get to the download page? Did you have to bounce around three different "upgrade now for only $19.95" screens? Did it ask you to use their special download manager? These are all warning signs that the software is likely crapware.

Further to that, if you've downloaded a piece of software and are ready to install... take your time going through the installation steps. Read each page carefully to ensure that the Agree button is not going to install a search toolbar or some free extra crappy program. And scan each screen for tick boxes that might make mention of changing or doing something that you don't want it to do.

If you need an example, just check out the insidious download page for Adobe Flash. Right there... Right in the middle of the page is a big fat tick in a box that most people would ignore, but will end up having Chrome installed without them even realizing. It's odd, it used to be McAfee, wonder why they changed.

Anyway, that's the sort of thing you need to be looking out for.

Don't trust any program. Always check what you're Agreeing to.