As long as you still have a copy of the Admin Console CD (as Sidewinder G2 went end-of-life almost 3 years ago, I'm not sure if the Admin Console image is available for download any more), you should be able to install it on any Windows 2000 or XP machine.
The Admin Console itself has no license, so you can install it as many times as you wish, wherever you want.
Any issues you are likely to have will be with Vista and Windows 7 (aside from it not being officially supported, from what I can recall). The introduction of user access control security meant that once you installed the admin console and tried to connect to the Firewall, if there were outstanding admin console patches to install the automated installation process would not work. In some cases, when helping my own customers, I found that disabling UAC for the duration of the exercise did allow this upgrade process to work - but not in each case (I don't know why).
You could get around this by copying the AC3xxx.exe files from the /var/spool/ACpackages directory on the Firewall and then run each executable by hand on the new machine to bring the admin console in line with the version of Firewall software running on the appliance. It will work, as I still have a functioning copy of the G2 Admin Console running on my Windows 7 PC.
Hope that helps.