If you just want San Andreas covered in snow, then here's what you should do.
Find a way to get the gta3.img file with Snow Andreas in it (do not get the gta3int.img file or whatever it's called, since it changes ALL computer screens in-game to porn... city hall, LSPD, et cetera... but for those pornfreaks here, well...

) and use it to replace the gta3.img in an unmodified copy of SA. Voila.