Windows 10 may be free, but only if you’re already running Windows 7 or Windows 8, and crucially, only until July 29. If your PC still uses Windows XP or Vista, you’ll need to pay for the upgrade and that’ll set you back around £99 from Microsoft.

That’s a big disincentive if money is tight, but sticking with those two old operating systems isn’t very sensible. As we’ve mentioned many times before, it’s unsafe to use Windows XP now that it’s been abandoned for over a year and it won’t be long before Windows Vista is in a similar state — Microsoft will abandon it in April 2017.

So what are your options? Let’s take a look.

[Related story: The clock is ticking for old versions of Windows]

Buying Windows 7 second hand

If you want to stick with Windows rather than, say, Linux, then you might be tempted to buy a second-hand copy of Windows 7 or 8. These are widely available and perfectly legal after the European Union Court of Justice ruled in 2012 that software could be resold — despite protests from software companies.

Ebay is the obvious source of second-hand anything and software is no exception. Windows 7 install discs, for example, start with a ‘Buy It Now’ price of around £5 and that sounds like a bargain — but beware.

Beware of those bargains

Those bargain basement Windows install discs are almost always ‘repair’ or ‘restore’ discs that were originally supplied with a new PC in lieu of a full Windows install DVD. That means they may not be able too install Windows at all, but simply ‘restore’ a PC’s hard drive to a factory fresh state and that could end up wiping out all of your hard drive partitions, rather than the one you just want Windows on.

Windows restore disc

That’s if such a disc works at all, of course. Some will only restore Windows to the make (and often model) of PC they were originally supplied, so they’re not much use to anyone else.

The importance of a product key

Genuine Windows 7 and 8 install or reinstall discs tend to cost a bit more, but pay close attention to what you’re buying.

A Windows install disc is no use by itself and it also needs a valid licence and corresponding product key to be legal to use. That product key is the important part, since without one, you won’t be able to activate Windows once it’s installed. That means both Windows 7 and Windows 8 will stop working after 30 days, and while you can reinstall either to start again, that’s not something you want be doing every month.

Install discs that have been opened are likely to have been used and even if they come with a product key, there’s no way to tell if it’s already in use on another PC. If that’s the case, you probably won’t be able to activate it on your PC and Microsoft may not be sympathetic if you get in touch to explain the problem.

[Related story: Why you should upgrade from Windows XP sooner rather than later]

Why a COA is key

So that safest option for buying a second-hand copy of Windows 7 or 8 is to look for one that comes with a ‘COA’. Short for ‘Certificate of Authenticity’, this is essentially a genuine Microsoft licence and product key, and while both can be faked, it’s a criminal offence to do so and that’s obviously not in the best interest of business sellers on eBay.

Windows COA

These install discs tend to come from PCs that came with a different version of Windows (often Windows XP) and that have since been scrapped. Read the seller’s description carefully to make sure you know what you’re buying and check their feedback, too, but around £30 should buy you a legitimate copy of Windows 7.

Windows 7 ‘refurb’ discs offer a similarly low-cost option. While intended for use with refurbished PCs, they will still work with other computers and prices start from around £29.90 online.

Windows COA for sale

Buy a new PC for £15 more

If you bought your PC new when Windows XP was still a going concern, then paying £35 to upgrade it to Windows 7 doesn’t make much sense. Unless you’ve upgraded its hardware in the meantime, it may not be up to running Windows 7 and a new PC is almost certainly a better option. That’s ‘new’ to you, of course — the PC itself doesn’t need to be.

Refurbished PCs complete with Windows 7 can be found for as little as £50, so you’re only paying £15 more than a second-hand copy of the operating system itself. And as long as you buy from an authorised dealer, you’ll get a warranty to protect your purchase, too.

Refurb PCq

Specifications won’t be great for that sort of price, but they’re likely to be better than your current PC and you may even get a new monitor as part of the package. Amazon has a wide selection of refurb PCs, but be sure to check the customer reviews before buying.

Read more: If you insist on keeping Windows XP, the least you could do is use the safest possible web browser. Find out the best here.

Are you an XP user considering upgrading to Windows 7? Let us know in the Comments section below.

Updated by Jamie Harris on 13/07/2016