Windows and on line banking

I have just set up new computer which runs on Windows 8. My old computer uses Windows 7. My problem is:
I cannot access BMO online banking with new computer but it will work with old one. BMO cannot help. I get an error message like "unable to access your account at this time" and directs you to call MasterCard. I have called them but didn't get any help as they do not have any tech support. The problem must be related to W8 or something I have done incorrectly on my system. But I can access Visa on new system