assuming you have your computer adequately protected against viruses and malware, etc. the thing you need to worry about is "man in the middle" attacks. do a google for an explanation.

to guard against this you should use an institution that uses a confirmation check each time you login. for example it asks you a "secret question" that you've answered when you setup your account. or, it shows you an image at each login and you recognize that image each time you login. a man in middle attack won't know the answer to the secret question, or, in the case of the image check won't know what to display.