I think you can attack under heavy rain, but not 100% sure about it either.
From the manual: "The number of strength points used for attack is also halved [...] by overcast weather for air units (air units may not attack in rain or snow)." Except the attacking unit has the leader All Weather Combat.
Also, I did a virus scan on my PZ2 and all of a sudden when I click the icon on the desktop to play it tells me that it wont play because the file "SMACKW32.DLL" is missing and I have to try to reinstall...how the heck do I do that? Just download the game again from the site after uninstalling or don't uninstall and just download or try to download that specific file or just give up because I have had nothing but problems with this game on Windows!
Sounds like your anti-virus program didn't like some old packaging methods and put the file into quarantine.