I have my steam program folder on a shared partition between XP 32-bit, Server 2008 64 bit, and Windows 7 64 bit (trial). I run the existing steam executable within its program folder on all OS's without problems. When I delete or install an OS, I just format the partition that the OS was on, but leave the large, shared partition with steam on it untouched. If there is something missing that is needed when first running steam or any steam apps in a recently installed OS, steam installs it. The only thing you really need to be aware of are game saves. I can't seem to share my Dawn of War 2 saves, for example.