Step 5 can be run on Windows systems, but the older versions have to be run in a DOS box. I use Step 5 V3.0 in a DOS box on both Win2000 Pro and WinXP Pro. The versions from V6 on are intended to be used on Windows directly.
A good third-party alternative is S5 for Windows by IBH-Softec. A demo can be downloaded there. There are others which also provide alternatives for the rather expensive original Step5. Search this forum and you'll find these.