|Windows Compatibility Mode|
This section holds any information about how to indicate what operating systems a product might be compatable with (under WIN2000+), for example an application running under WINXP might need to be told to run as if it were running on WIN98.
If you wanted to ensure that an application only installed on certain operating systems then you'd use approprate launch conditions.
Untested by me but apparently the following works:
[HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers] "C:\\Program Files\\WhereEver\\My95App.exe" = "WIN95"
Some good links: