One in four Irish companies is expected to have Microsoft's IT security support withdrawn in the coming months, unless they buy new computers or pay extra fees.
The tech giant will stop protecting Windows 7 from viruses and malware from January 14, except for those who pay a new premium for extended security support.
Two years ago, Ireland faced a ransom epidemic, in part because of outdated Windows software on PCs.
Malware, including Wannacry, shut down HSE services and caused many small businesses to pay criminals hundreds or thousands of euros in cash to unlock their computers. Within three months, hundreds of thousands of Irish PCs face potentially similar vulnerabilities when their operating systems lose security support.
"Cybercriminals are very keen to take advantage of this," said Connor Flynn, one of the country's oldest IT security specialists.
"There is no doubt that we will see the impact of this."
"It will lead to attacks, whether it is a ransom or any other exploitation of computers because they will be vulnerable," said Mr Flynn, ISAS managing director.
Microsoft has warned that only those who pay a new premium for "extended security support" will be safe from the expected malware attacks.
This new annual premium fee costs between 50 and 100 euros per computer per year for up to three years.
Even though the deadline is only a few weeks, Microsoft has not yet revealed how much increased support for small businesses will cost.
"We don't have an answer for that yet," said Shirley Finnerty, a leading Windows Ireland business and device maker for Ireland.
The prices are expected to be revealed in December. Reports earlier in the year indicated they were briefing company customers on the likely costs.
For the Windows 7 Pro, it reportedly costs almost € 50 per device for the first 12 months, twice as much for the next 12 months, then double again (almost € 200) for the remaining 12 months.
So an organization with 1,000 PCs stuck using Windows 7 Pro could face a security fee of 50,000 euros in 2020, and it will rise to 200,000 euros in 2022 if they do not sort the updates in time.
The comparisons are approaching the chaotic back-end problems that came when Windows XP was cut from security support five years ago, when a wave of viruses and malware caused havoc.