Problems with Wise for Windows Installer.

Im hoping someone can give me some advice or some answers. I have a program that was coded in VB and then packaged using Wise for Windows Installer v4.21. I have sent out about 150 of my programs on CD's to different people. However, two people have received error messages when trying to install the program. The error is:
This installation package cannot be installed by the Windows Installer service. You must install a Windows service pack that contains a newer version of the Windows Installer service.
I belive that both users are using a windows 98 machine. Anyone know why they are getting this error when all the other people were able to install just fine?
Thank you in advance for any and all information you can help me with.
Thanks,
Bryan

I guess there systems installer is out of date. Maybe you could use the Package and Deployment wizard and send that setup to them. If both are Windows 98 then I'm assuming something must be wrong with their system.