I know I'm a little late to jump onto this thread but let me say not all public computer netowrk is unsecured. The key thing is that you need to know what to look for when doing a secured transaction. As a rule of thumb, never use a computer that is in the public domain for secured transaction, as you can never tell whether it's been rigged/infected. However the same cannot be said regarding public network. Of course you shouldn't really pay your bills online on a network offered in places like Starbucks, but if we are talking about a public network that is offered in say an university, the risk involved is just not the same. It all boils down to who and how much you trust, so in the case of using an university network you are putting your trust on the IT admin guys working in the uni to do their jobs right - updating their servers regularly, locking switches/servers/network equipments securely and have patrols regularly to ensure they aren't rigged etc, and trusting that there aren't some double agents inside the IT dept working for a malicious syndicate.
As a rule always use https, and try to use some browsers other than IE if possible.
For Wifi, again, it comes down to trust. If, say, you trust that the IT staff working in Harvard University are decent people and you trust Harvard's network (I know it sounds crazy, but if you want to get things done in this world you have to trust someone at the end of the day - I mean, would you trust the cops? Would you trust the traffic lights are working properly? If you're not trusting anyone just because of a simplistic reason that you want to keep safe then you can't get anything done), then the onus would be on yourself to ensure that you're indeed connecting to Harvard's netowrk instead of logging onto some networks that claim themselves to be Harvard's. The key in doing this would be to teach yourself about understanding digital certificates. If say, Harvard's Wifi access points are certified (I'm not sure if they are, because to have a CA certifying access points regularly is quite a financial undertaking so not every company, even reputable ones, does that), then you need to configure Windows to only connect to those that are certified. If you can't find an wireless access point that is in anyway certified, you need to always bear in mind there's a certain risk that the access points broadcasting in your area are impostors.
It's a sad thing that in many of the so-called developed nations today many people don't understand the basic workings of digital certificate, when the exact things that you need to look for are actually very easy to find.