Provided you ensure that:
a) any site you provide credentials to, or pass sensitive information over, is using a https/ssl encrypted connection;
b) your laptop is fully patched, running an updated AV product, and preferably a personal firewall (the built-in XP one will be fine for this purpose);
.....then in the scenario you describe you
should be ok. There are methods for circumventing the protection that SSL provides, and there are methods of attack that will negate the protection provided by AV/firewall, but your window of exposure should be sufficiently small to make most of these impractical to deploy against you.
Any 'public' wi-fi connection should be considered 'unsafe' (I use the word in the absence of something more appropriate, I'm sure you understand) and you should ensure that you're mindful of the risks that they pose when you're using them. This isn't to say that you should avoid them, or in fact that you should change your usage/habits, but you
should always be aware of the risks and make sure you're doing what you need to do to protect yourself
