Downloads: Pidgin Portable

Pidgin (formerly named Gaim) is a multi-platform instant messaging client that supports many commonly used instant messaging protocols. Pidgin is free software available under the GNU General Public License. This is a portable Pidgin version. Simply download, run the .exe to install it on your USB and you can use it everywhere! This is very handy if you are at school and would like to chat using your Instant Messenger. Pidgin only uses low memory and is amazingly fast. Give it a try. It's worth it.
Features:

• Multiple OS support including Windows and many Unix-like systems such as GNU/Linux, BSD, and AmigaOS (through X11 engine)
• Multiple protocol support
• Multiple account access support
• On-action automated scripts for contact status changes (called Buddy Pounces)
• Tabbed message windows for conversation management
• Grouping multiple buddies into a single "contact"
• Conversation and chat logging
• Notification pop-ups available through the Guifications plug-in and libnotify plug-in
• Built-in NSS support, offering client-to-server message encryption for protocols that support it.
• Plugins "Off-the-Record Messaging" and Pidgin encryption, offering end-to-end message encryption.
• Contact aliasing
• Spell checking
• Notification area integration[1]

Supported Protocols:

• .NET Messenger Service (a.k.a. MSN)
• OSCAR (AIM/ICQ/.Mac)
• XMPP (Jabber, Google Talk)
• Gadu-Gadu
• Internet Relay Chat
• MySpaceIM
• Novell GroupWise
• SILC
• Yahoo!
• Zephyr
• Lotus Sametime (previously supported by the third party gaim-meanwhile plugin)
• QQ (previously supported by the third-party OpenQ plugin)
• Session Initiation Protocol (SIP) (at the moment only with chat function)

I am using it and it is working perfect. Pidgin is my favorite Messenger . You can also use several MSN accounts at once without any problems