Package pre-requisites

Hi,
I wonder if there is a way to install or not to install a package depending on the OS of the maschine.
Example: I like to install Filzip only on Win2k maschine not on WinXP.

Any OS detection solution with <check type="registry" condition="exists" … > do not work because the result will be the same after installation.

It there any way to create a condition like "package requires registry path exists"  (or similary for registry values or files) ?

Thank you for any advice.

Markus