Maybe you've used a wifi at a cafe/station/airport/whatever and some clever computer person managed to syphon off your details as you used the connection ? Maybe it's within the banks security architecture and you were just one of the unlucky persons who got caught in their net.
Probably time I went down the mac route
I would recommend that, but these days the uptake of users of Apple's products have made them mainstream and the hackers have taken notice. That said, touch wood, so far no real problems experienced in my case.
SHJ