From MSKB Article # 330117:
Windows Messenger is included with Windows XP and is installed when you install the operating system.
MSN Messenger 5.0 is included with MSN 8.0 and is also available as a separate download. It is installed on your computer when you install MSN 8.0 or when you run MSN Messenger 5.0 Setup. If you are running Windows Messenger 4.x, and you previously installed the MSN Add-in for Windows Messenger, you may receive a message to install MSN Messenger 5.0. If you did not install the MSN Add-in for Windows Messenger, you are not prompted to install MSN Messenger 5.0.
Please review the KB article referenced for important information regarding running two versions on Windows XP.