You have to change the username and password on the Windows 98 PC to the ExACt SaMe as what you use to log on to the XP machine.
Or...
You have to setup a user with sharing rights on the XP machine to MaTcH that of the Win 98 machine.
Either way, you will have to restart the Win 98 Machine
It has to do with user rights, and the answer is in the micoroshaft Tech-Net KB (just can't recall where)
EDIT:
Now that I think about it, I think I missed something...
You need to have both machines on either the same workgroup or domain (workgroup is simpler and easier)
Here's some good reading:
http://www.microsoft.com/windowsxp/expertz...pts/03Aug19.asp