If you ever need to backup your registration key (Transferring to a new computer, lost email with original key, etc.) you should be able to run the following on any version of Windows to backup your key:
regedit /e c:\soldat.reg "HKEY_CURRENT_USER\Software\Soldat"
If you need to, you can change where it's saved to by changing C:\soldat.reg to wherever you wish to save it to. You'll be able to then run the .reg file to register again when needed.