The easiest way to achieve this is to build a Windows XP Pro virtual machine using something like VMWare Workstation.
Once you have WinXP up and running you'll need to install Siemens Step5 v7.21. As far as I know this was the last version released.
As for connection from the PC to the PLC, that depends on which type of S5 it is. The 95 , 100 , 115 and 155 all use the 15 pin d plug connector. The very very old 150 series uses the 25 pin D plug, but I'd be amazed if you're working with that one, I think mine are the only ones left !
The best cable set on the market for usb to S5 is from Process-Informatik with Part No. 9359-1. There are a whole host of adapter cables that pair up with this cable to allow you to connect to all sorts of other S5 cards, like serial comms, ethernet comms, WF470 display card, and more.
I'm not sure if I'm allowed to post the link, just google it, you'll find it.
Bare in mind, what ever you come up with to gain access to your S5 PLC, what you upload from the CPU will be bare code, there will be no symbols, comments or any engineering text of any kind. A good start would be an electrical drawing detailing basic I/O to the PLC.
Good luck, shout if you need any help.
