During windows update, laptop restarted and later it went to a BSOD. The error code is: ‘IRQL_LESS_ OR_NOT_EQUAL_TO_ZERO’.

The only reason I can think of for this issue is a possible viral attack when surfing the Internet. As your PC is old, it could be possible that bundled/installed antivirus might have expired. PC manufacturers generally bundle an antivirus with the PC for free, but they are usually either valid for three months or a trial version. Hence, your antivirus might not have been securing the PC and a virus or Trojan might have entered/attacked your PC. Depending on the type of Trojan or virus, the PC might misbehave, throw up errors, underperform, or randomly crash. These activities are usually blocked by antivirus utilities and firewalls, which might not have been active in your case. Henceforth, use a good antivirus (either free or paid) and always keep both the antivirus and the operating system updated to prevent viruses and Trojans. Make sure to backup your important data at least once a week.

BSOD can be caused due to virus, faulty RAM, bad hard drive, loose cabling, corrupt software, etc.