Helping people with computers... one answer at a time.
While Windows Messenger is really only available for Windows XP, there are other messaging clients by Microsoft that will support Windows 98.
Where can I download Windows Messenger for Windows 98?
Windows Messenger is really only available for Windows XP or later, though there are versions that will run on Windows 2000. In looking into it, the Windows Messenger site itself even points elsewhere for a Windows 98 solution.
As I've discussed before, Microsoft puts out more than one instant messaging client. Windows Messenger comes with Windows XP and is available for download as well. There are versions that can be downloaded that work with a couple of additional IM protocols (mostly Microsoft's own Exchange Instant Messaging), and those happen to run on Windows 2000 as well.
But when it comes to Windows 98, Windows Messenger politely steps out of the way.
This page about Instant Messaging (IM) Clients for Exchange 2000 Instant Messaging Service includes a table that details much more, including which version of which client you want for what type of application. And on what operating system.
The bottom line?
If you want to chat with folks on MSN, use MSN Instant Messenger It claims to support Windows 98.
If that doesn't work for you, perhaps a third-party IM client will. Trillian is one client that should work with MSN instant messenger, and also claims to support Windows 98. My only hesitation with 3rd party applications is that they are not officially supported by the .NET messaging service, and if the service changes, they can stop working without notice until they're updated to match the changes.