If you're building a new PC
But what about if you're building a new PC from components? In most cases, those of us who build our own PCs need to individually buy our own operating system licenses. Further, to take advantage of the $14.99 upgrade sale, you have to supply an existing Windows 7 license code and retailer name, so an off-the-shelf OEM version of Windows 7 might not be eligible.
So, for this recommendation (and all the rest in this article), I'm assuming you're making the decision based on the $39.99 Windows 8 Pro upgrade offer. The other factor is that this $39.99 price is an upgrade. You're going to need to sacrifice an existing OS license to be able to perform the upgrade.
The fact is, no new PC should be running XP or Vista. XP is just too vulnerable and support is ending in 2014, and Vista is, well, Vista. So you should either install Windows 7 or Windows 8. Also, it's getting far harder to find workable XP drivers for new components, so you're pretty much forced to go with Windows 7 or later for production use.
From a cost point of view, one approach would be to recycle an old XP or Vista license on your new PC, get it installed to the point of basic operation, and then install the Windows 8 upgrade over it for forty bucks. That will get you a modern OS on your PC for a pretty inexpensive price.
On the other hand, your decision probably should be made more based on which drivers are available for the components you've chosen. Go to each vendor's driver download site and be sure there are drivers available. The OS with the better drivers is probably going to be your choice.
If you're running XP
This is perhaps the easiest recommendation, but even here, what you do depends on how you're using XP. I'll talk about the edge cases in a moment. For now, let's just discuss those legion of XP machines running around, open, and completely vulnerable on the Internet.
I'll put this quite simply: if you're running an XP machine and you're reading email or browsing the Web, you need to upgrade immediately. XP is enormously vulnerable to exploits, you can't run Internet Explorer later than IE8, and your PC is a calamity waiting to happen.
So you should upgrade to either Windows 7 or Windows 8. Period. Now, as it happens, it's probably actually a better choice to upgrade to Windows 8. First, again, you can get the Windows 8 Pro upgrade for only $39.99 and any Windows 7 upgrade will cost you more.
Second, as Jason Perlow reports, Windows 8 can breathe new life into old PCs and provide better performance and boot-up speed than Windows 7.
Essentially, upgrading an old PC from XP to Windows 8 will give your PC a new lease on life. Otherwise, you're probably living on borrowed time.
Now, what about those edge cases? What I'm talking about are old Windows XP machines that are single-use devices and don't browse the Internet. For example, I have an old server monitor that runs one piece of software -- a display of which servers are up or down -- and I think I rebooted it once, back in 2009 or so. For PCs running XP that are special use devices, I don't necessarily recommend upgrading. Especially since some of that special-use software might not run on Windows 8 (although you could run it in a Hyper-V XP-based virtual machine, another cool Windows 8 feature).
But, for every other XP user, I strongly recommend upgrading to Windows 8.
Oh, and as a shout-out to all of you gung-ho Linux kiddies, I will point out that my ZDNet colleague, Steven J. Vaughan-Nichols, colorfully claims Windows 8 belongs on older PCs like a fish needs a bicycle. He recommends installing a Linux distro. Of course, Steven pens the Linux and Open Source column, so take that into account when making your decision.
If you're running Windows 3.x, 95, 98, or Me
Seriously? You are seriously still running one of these antiques? First, Windows 8 won't upgrade one of these clunkers, and second, if you're still running something from the Cretaceous Period, you probably have your own twisted reason.
Frankly, if I were you, I'd rush down to my local store and buy anything else, but perhaps you're living in the Museum of Forgotten Toys. Anyway, I can't help you. Not sure if anyone can.
Next up, if you're running Vista, Windows 7, and my recommendation summary...