We've covered the topic of Web browsers, but we haven't covered Instant Messengers yet. If your going to be using a browser, Meebo.com(Secondary Post) is a good place to start. It can connect you to your AIM/Yahoo/MSN/ICQ/Google/Jabber account from the page. If you sign up for an account you can sign in using a username/Password and access all your IM accounts at the same time. And it works very well for a web app.
Another option is to try Gaim/Pidgin. While Gain is now a thing of the past, the portable version runs very nicely. It even encrypts your stored passwords should you save them to your USB device. Portable Gaim may work well enough as it is, it still lacks all the portability we need. Most file locations are not saved to relative paths which can makes us sad. Because not all machines will list your USB drive to the same letter (I know a place that will list it under D). But if you need something right away, Portable Gaim could be just what you need.
I'm testing out Pidgin at the moment. Even as this post is being drafted I'm running a portable install of Pidgin. There are a few people who have "built" their own portables for Pidgin but it seems some have keyloggers bundled with them(not good). I looked over the Official Pidgin Website and I have found some instructions for making Pidgin portable. As I wrote, I'm testing said build right now. The people behind Portable Gaim haven't made their own "Official" build either but I don't think we need to go to them for one anyway(Haller-PX).
So for now Use Meebo and Gaim(portable).