Not knowing any further details, if it's a Windows machine (assuming it is, because you said "desktop") it sounds awfully like a corrupted Windows Update process. I've had this happen on several computers and the fixes have run the gamut from "very easy" to a day on the phone with Microsoft.
It could be the "anti-VIRUS software." If it is a Windows machine, you could try to start in Safe Mode and uninstall it (i.e., the anti-virus software). Since it's expired you would get the benefit of a fresh install using - may I suggest - Malwarebytes.
As far as it being 3-4 years old, that shouldn't be a problem; I'm still using MacOS and Windows machines that are far older.... Of course, if the OS is Windows 7 or earlier Microsoft is going to tell you it no longer supports those OS (MS still supports Windows 8, 8.1, & 10).
I would call Microsoft before I called the desktop manufacturer. The MS rep can help (even if only to tell you it isn't a Windows issue or your Windows OS is no longer supported, etc...). That will give you a good idea where to go/what to do next.
Best of luck!
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