The "serial number mismatch" error when installing in Windows 98SE or ME is fixed in Microstudio 2.609.
If you have Microstudio 2.5xx or higher, find your download link email, click the link to get your free download if 2.609. When you install it the error should no longer appear.
If you have Microstudio 2.4xx or lower, you need to purchase the upgrade for $15 to get 2.609.
Thanks for your patience through this problem with Windows.