Esgrimidor:

If you are talking about the registry backups take during installation or by going into Spybot » Mode » Advanced mode » Settings » Settings » and then clicking on the Wizard button, there are two files created as back-ups during installation:
  • regLocal.reg (which appears to the [HKEY_LOCAL_MACHINE\Software] registry key).
  • regUsers.reg (which appears to the [HKEY_USERS] registry key).

The directory they are created in depends on the OS you have as follows:
  • Windows 95 or 98:
    C:\Windows\Application Data\Spybot - Search & Destroy\Backups
  • Windows ME:
    C:\Windows\All Users\Application Data\Spybot - Search & Destroy\Backups
  • Windows NT, 2000 or XP:
    C:\Documents and Settings\All Users\Application Data\Spybot - Search & Destroy\Backups
  • Vista:
    C:\ProgramData\Spybot - Search & Destroy\Backups

However, those backups are relatively large portions of the system's Registry and I personally would hesitate to use those backups to restore the Registry (at least in their entirety). Once you've run your system for any period of time after they were created they would be outdated and could possibly do has much harm as good.