A. This is not actually a Regclean problem but rather just an association problem between .reg files and regedit.exe. To resolve perform the following:

Use the following steps to correct this problem:
  1. Start Explorer
  2. From the Tools menu select 'Folder Options'
  3. Select the 'File Types' tab
  4. Find "REGĀ  Registration Entries" and select it
  5. Click Advanced
  6. Under Actions select 'open' and click Edit
  7. Change to:
    regedit.exe "%1"
    and click OK
  8. Click OK to all dialogs

You should now be able to run the undo file by double clicking on it.