For those whom computer can run Windows 7 & installs it, or buys one with it or Windows 8 pre-installed, but prefers to keep the feel of XP, there is Classic Shell & it's free.
Though I don't use it anymore on Windows 8 or 8.1, I once did & recommend the software to help those who are having a hard time adjusting.
As to refurbished/used computers, proceed with caution. I once bought what was described as a refurbished laptop on eBay, complete with Windows 7 Pro & Office 2010 for $400. Only to find out 4 months later after a Windows Update that it wasn't genuine. After rebooting, was greeted with a black screen with the watermark that I may be a victim of pirated software (or similar wording, this was in 2010).
Fortunately I had another of the same exact laptop years earlier (Dell Latitude D610) & though the MB went bad, I saved the software that was supplied with it, including the Dell OEM XP Pro reinstall CD. It installed & activated perfectly.
Knowing what I do now, unless one truly knows the previous owner, one of the top 10 worst purchases that can be made is of a refurbished/used laptop. For starters, there is no way to know how it was treated/cared for. Many lays them on carpet to do their work/browse the net, this only serves as a mini vacuum cleaner, drawing the dust from the carpet into the laptop. This in turn clogs the heatsink, restricting airflow. It then starts to run hotter & louder (the fan will eventually run wide open).
Even if the installed version of Windows is legit, the purchasing of refurbished laptops is risky at best. Large retailers & discount clubs are always running promos on brand new laptops & PC's for as little as $288, which will do for someone who only browses the net, makes transactions & checks email. For those who needs more power to run virtual machines & photo editing, another $200-400 goes a long way. Only the most serious of gamers needs the ultra expensive models. One can easily add a SSD & extra RAM for blazing speed at a fraction of what the OEM will sell it for, plus you get to keep your original HDD for backup/data. The OEM keeps your "base" HDD & installed RAM & recharges you for the upgrade(s).
Fairly much the same with desktops, is dealing with the unknown. If a deal sounds too good to be true (such as a 2008 model PC loaded with Windows 7 Pro/Office 2010/Full Adobe suite) for a low amount of cash, your feeling is right, chances are it's not legit. Stay away.
Times has changed, computers like other electronics has became more modern, are lighter, uses much less energy & most will allow for at least 8GB of DDR3 RAM (many ships with that amount). And the good thing about buying new versus refurbished is that it already has been certified to work with Windows 7 or 8/8.1. The deals are there, one simply has to search for them, keeping in mind to consider what you use your computer for. Don't buy a computer that is underpowered for your needs, you'll soon regret it. Fortunately, one of the major warehouse clubs in the US (Costco) allows for 90 day returns on computers/other major electronics. Buy from reputable sources.
Keep in mind when you're shopping online at the OEM "outlets", where many "open box" computers are sold at a small discount, that chances are there was a legit reason why the customer didn't want the computer, or it's one that someone bought & it's a "factory refurb". That's only slightly better than 3rd party refurbs, many only has a 90 day warranty, again not worth the risk. You have a great chance of having the same issue as the original owner, only to find out after the 90 days are passed. OEM's tend to overcharge for their products anyway.
As a final option, if your computer is still in good running condition & cash is tight, consider a version of Linux. Linux Mint/Ubuntu are two of the most used Linux based OS's & runs great on many computers. However if your XP powered computer is really old & doesn't support PAE, Mint 13 is as high as one can go. Still, that's support until 2017. Later models, especially those that shipped with Vista & many downgraded to XP, can run most any Linux distro. Speccy is a free tool by the writers of CCleaner, it'll show if the CPU is PAE/NX enabled.
XP's out of support date is less than one month away, so I felt it best to post something in regards to it. Just don't rush to buy that refurb because a site says they'll soon be all gone. They're lying, there's been a refurb market ever since I began working with computers in 1993, it's here to stay. There are already refurb Dell XPS 8700's that cost more than mine did when new last October, some much more so & mine is i7 equipped with 12GB of DDR3 1600MHz RAM. Bought new for $699, $350 less than Dell's price for the same setup. Some refurbs are already going for over $1,000 on both Dell & eBay. Suckers are born daily, don't be a statistic.
Though this has been stated before, I'll repeat it again, it's not safe to keep on using XP after end of support, just because security companies supports the OS. They do this for money, not as a favor to you. Some things, security solutions cannot protect one against. Such as unpatched code that has been patched, retreaded, & band-aided multiple times, with a final patch on April 8. The XP OS of today is similar to those old TV shows (comedies) that exposes old plumbing, with rags around the pipes, pots on the floor, homemade joints, you name it, anything except replacing to repair leaks. It's time to repair the issue by upgrading, not further repairing.