To tam přece píšou hned na začátku:

NOTE: If your EXE file associations are corrupted, it can be difficult to open REGEDIT, or to even import REG files. To work around this, press CTRL-ALT-DEL and open Task Manager. Once there, click File, then hold down the CTRL key and click New Task (Run). This will open a Command Prompt window. Enter REGEDIT.EXE and press Enter.