Downloading Windows 98 Second Edition without a license is illegal. The license contains a string of numbers nessecary to complete the installation process. It's one license per computer.
However, with only one license, there's nothing stopping you from installing a copy onto multiple machines. But technically, you're not supposed to do that.
If you are thinking about buying Windows 98 Second Edition, why don't you check on websites like eBay or TradeMe? The discs can be obtained for pretty cheap there, and you won't be breaking any laws either.
Vetusware offers free downloadable copies of Windows 95 but I don't think that's what you're looking for.