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Obviously this consideration decreases the size of the pool considerably, since Windows 98 by that stage was getting long in the tooth. To judge by W3 Schools statistics (h/t to Lance at Catprint.ca), only 12% of users world-wide were using Windows 98; the same site (again h/t Lance) reports (here) that 69.7% of users were using IE 6.0. Taken together these imply something like 10% of all internet traffic used this combination.
Two quibbles. First, this was 2003, when the Canadian dollar was at $0.63, which may have produced a drag on hard-ware replacement and thus meant that Canadians lagged a little in the move away from Windows 98. Second, the browser statistics are browser-use for all Operating Systems, not only those using Windows 98, for whom some of the browser options listed there (Mozilla at 6% and Opera at 2%) were probably little used among the old system. Both these considerations should inch the total upwards slightly. But 10% will not be far wrong.
So, 10% of 700,000 Rogers customers in Ontario is 70,000. (Another quibble that may force this upward -- I'm not sure that I am correct to limit this to Ontario customers. I suspect, but cannot demonstrate, that customers in Quebec, New Brunswick, and Newfoundland were also proxied through these three banks)
(Interesting side-note: these old systems are not completely gone. I had two visits yesterday from users who had the Windows 98 and IE 6.0 combination.)