Is that Windows 7 32 bit or 64 bit. I'm absolutely sure that Siemens Step 7 does not run under 64 bit. In fact the upcoming Portal won't run under 64 bit windows, at least at first.
I don't think there's a problem running Siemens in 32 bit W7 ? At least not the latest version 5.5 (and I also think 5.4 was fine).
But if you are unsure you could download VMWare Player for free. Windows 7 comes with the ability to run XP as a virtual machine. You can use VMWare (other virtual machine software applications are available) to run XP and then install Siemens in your virtual XP environment. I've done this with S5 and it works a treat.