If my VISA card no. is 4567 0003 1966 2924, I type something like 5674 3000 6619 2429, and then CUT and paste, ALTERNATING between the mouse and control-X and control-V, to put them back into the right order. I've only found one website which doesn't allow you to do that, so guess what? Right. They lost a customer.
Since new and ever more clever trojans and spyware are invented every day, your use of online banking and business with critical data should be limited to whatever you're DEAD SURE cannot be cracked. On PCs that's VERY limiting, IMHO.
Thank you!