Microsoft Java Virtual Machine vs Sun JVM

I have an app that was written when MS JVM was in place and when users install SUN JVM the app stops working.

This usually happens when end users upgrade their OS.

I know this is very limited information but does anyone here know how to enable one JVM over the other ?

Also, if anyone here has any ideas to do with upgrading from
IBM HOD v 3.02 + ResQ!Net 3.2 to HOD v7 please let me know.. I am in desperate need of some information.

Thanks,

Alvin