Helping people with computers... one answer at a time.
When you have a serious or semi-serious problem with your machine, you're faced with several options for recovery. I'll explain and compare a few.
The scenario is that something 'ain't right' and I have the option of 1 - format & reinstall: 2- manual repair. 3 - run sfc(/scannow). 4 - use a reimage site. I can understand what option 1 is, but what are the pros and cons of options 2, 3 & 4 and how do I know which is the best option(s) for any given situation?
What you've run into is a very common situation people find themselves in from time to time.
It boils down to "what the HECK do I do?"
I'll run down the options you list (at least the ones I understand), and throw in two more.
Format & Reinstall is perhaps the most extreme and difficult of the lot. It involves erasing everything on your machine by reformatting the hard disk, and then reinstalling Windows, every application you use, and your data. Typically, the "your data" part is best handled by taking a full backup of everything prior to the erasing everything, so you have a copy of all your data at the time that you can copy from.
The downside: it's a lot of work, and it requires that you have installation media not only for Windows, but every application that you use. And it requires that you have some way to backup your data for restoration after.
The reason I often recommend it is that it's often the only way to make certain that malware has been removed, while at the same time cleaning your system of the cruft that's accumulated over time to slow it down or make it less stable.
Manual Repair (at least the way I'm going to interpret what you mean) is at the other end of the spectrum. This focuses on identifying a specific problem your system might be experiencing, and determining the steps to repair exactly and only that symptom. Running an anti-virus program to remove a virus, updating a driver, or going in to the registry to make a specific change might all be examples of a manual repair.
A manual repair is most appropriate when the problem is clear, isolated, and you know what you're doing. Don't read too much into that last part - "knowing what you're doing" could be simply knowing how to take a symptom, research a solution, gain some confidence from the research that says the proposed solution is correct, and then implementing that solution. How much you need to know depends on just how technical all that may get.
The problem with manual repair is sometimes you might find it to be incomplete or downright wrong. It can be a time consuming process to try a repair, find that it hasn't worked, and then start the research process again and again.
SFC /scannow is the System File Checker. It's a fairly simple utility that checks to see if your operating system files have been damaged or compromised, and fixes them if so.
It's simple, and there's rarely any downside to running it.
There are two problems with SFC: it only works on Windows system files, and it's probably going to require your installation media as updated for the service packs you have installed.
SFC is the System file checker, if there are problems with non-system files, like applications or other files, SFC can't tell you. In order to recover damaged files, SFC may need to ask you for installation media. If you've installed service packs for the operating system after having received your media, you may not have the media that SFC might require. (It first attempts to use updated files on your hard disk, but if those aren't there it has no choice but to ask for the original media.)
Use a reimage site - to be honest, I'm not sure what you mean by this, or specifically a "site" in this context. So I'm going to address backup or imaging software instead.
One of the arguments for taking regular full or "image" backups of your machine is that you then have the ability to restore your machine to the exact state it was in when that backup was taken. Think of it as a snapshot in time.
In my opinion, it's the best of all worlds - it can let you recover from almost anything, much like a complete reformat and reinstall above, but without having to reformat and reinstall.
Recovering using an image erases, in a sense, everything that's currently on your machine and replaces it with everything that was on your machine at the time the image was created. Everything.
The biggest downside is that it's not something you can do after the fact. You must have been taking images and backing up while your system was working, in order to have a good image to revert to when things go bad. As I keep repeating: backups are good and necessary and important, and this is one reason why.
Repair Windows is something you didn't mention, but something I want to include. Normally, when we think of reinstalling Windows we think of it in the "reformat and reinstall" context: everything is installed from scratch. Windows also allows you to perform a "repair install" which does in fact reinstall Windows, but without destroying most of your settings, installed applications and data.
It's a good thing to have in your arsenal of troubleshooting techniques, particularly if you have no backup (take one first, just in case) and a reformat and reinstall is impractical.
To be clear, it doesn't solve everything - there are many classes of malware that it simply won't touch, but on the other hand there are many that it may successfully eradicate. And it also requires that you have the proper installation media in hand. (And once again, if you've installed a service pack since receiving your install media, the repair may not be possible from that media.)
System Restore - I'm mentioning this because while it has occasionally helped people with more focussed issues, I want to reiterate that it's not something that can be relied upon, and it is no replacement for backups.
But sure, give it a try, it might help also.