Q: An acquaintance urged me to get the free Windows 10 update for my desktop PC, an Acer Aspire X3910 model that I bought at the end of 2010. But the upgrade failed and my PC was restored to Windows 7. What can I do?

Barb Berggren, New Brighton

A: Windows 10 failed to install because your 2010 PC is too old to handle it. If you really want the upgrade, you'll need to buy a new PC.

But your situation isn't unusual. Many PCs currently in use, including some sold as recently as the first half of 2013, are technologically too old to use Windows 10. Fortunately, several PC makers have made it easy to find out whether your particular model can handle the upgrade.

In your case, I saw that your Acer model did not appear on the company's list of Windows 10-eligible PCs (see tinyurl.com/jztu68l). Readers with other PC brands can check on Windows 10 compatibility at these websites: HP (tinyurl.com/o79he3r and enter model number), Dell (tinyurl.com/jy27jky), Toshiba (tinyurl.com/q3qylhj), Samsung (tinyurl.com/plyv75f and enter model number), Lenovo (tinyurl.com/oa94qpv) and Asus (tinyurl.com/pazg34t and enter model number).

Q: I'd like to keep Windows 7 as long as possible rather than switch to Windows 10. How long will Microsoft support Windows 7?

Tobey Hyman, Pembroke Pines, Fla.

A: Windows 7 will be safe to use until January 2020 because Microsoft plans to provide security updates for it until then. However, there won't be any new features added to Windows 7 because the company stopped providing improvements for it in January 2015.

Q: We have two PCs, one with Windows XP and the other with Windows Vista, and we'd prefer to keep using them to avoid the cost of new PCs and software. Will using Norton security software keep our old PCs safe?

Bill Sandras, Colorado Springs, Colo.

A: No, Norton and other security software can protect you from general computer threats, but not from attacks on internal flaws of the Windows operating system. New security flaws in Windows are found frequently enough that Microsoft provides Windows security update software to protect you against them.

The problem you face is that Microsoft has discontinued security updates for Windows XP, and next April it will do the same thing with Windows Vista. Without these security updates, your two PCs won't be adequately protected, and should be replaced by Windows 7 or 10 computers.

That said, there is a workaround for XP security, but it's risky. By making a simple change in the PC's registry, or database of settings, you can keep getting Windows XP security updates from Microsoft until 2019. But the updates are tailored to ATMs and cash registers that run XP, not PCs, so it's uncertain whether you'll be fully protected. For details, see "Tweak the Windows XP Registry" at tinyurl.com/j47lzrd.

E-mail tech questions to steve.j.alexander@gmail.com. Include name, city and telephone number.