Computer Programs, or applications (Apps) can make or break a computer system's usability. Here's a rough guide to making sure you can squeeze as much use out of one as you can. Language used will be as general and system agnostic as possible, with exceptions for system specific quirks that I know.
Computer systems can range in capability, which makes a one size fits all guide incredibly dificult to produce. So, let's get our expectations out of the way in order to have a more reasonable amount of things to consider when dealing with a variety of different setups. Our expectations of a system to be reasonably usable are:
With these requirements in mind, let's consider the options for operating systems alone, as that is the base for everything that goes on in the computer. There are several options, with varying levels of security, ease of use, program support, hardware intense operations, and stability. I will provide another list below, containing the possible options.
PowerPC has not been in use in personal computers for a very long time by this point, the most powerful usage of the architecture I am aware of being the G5 Macs from Apple. These are very old and have little software support, due to being completely different than the x86 architecture. Likely, if you want to use one of these systems, an obscure linux distrobution is your best bet. You may have to compile most software yourself in that case, excluding any closed source software support from what you can do, and the power consumption that would take alone is probably simply not worth it for the average person. Overall I just don't see these systems as a good solution.
Linux may be an iffy option for trying to get the most use you can out of an old computer. This isn't so much because once you get it booted it wil have issues, it's actually because in our requirements you should have noted that I didn't say the processor had to be 64 bit, and this is Where the issue arises. If you are using a 32 bit only sytem, like a netbook or an early "Vista Capable" laptop, a lot of Linux distros like Ubuntu and it's derivitives simply don't support it.
Likely, if you're running old (End of Life) Windows, you've accepted a few things. Namely, that exploitable pieces of the OS itself can be taken advantage of as soon as they're found, and are not going to be fixed, leaving you at risk for a simple mistake to completely compromise your system. Another thing is that 3rd parties have already stopped supporting these systems directly, in the case of Windows 7, 8.1, and even longer ago xp and Vista. In the case of system drivers, since this article talks about using older systems anyway, the only issue that would arise would be if they're nowhere to be found on the internet. In the case of web browsers, there are actually quite a few options, most even support xp, but I wouldn't reccomend it because there are fundamental problems with connecting an xp machine to the internet that have been found relatively recently. As for old MacOS, the only thing that makes it more secure is it's obscurity. Do with that what you will.