This is because your anti-virus software is preventing this operation.
Try the following workarounds:
- Open the files by double-clicking on them in Windows File Explorer;
- Add the Viewer folder (default path is C:\Program Files (x86)\Lizard Safeguard PDF Viewer) in the list of “don’t scan this folder/file” in your AV settings;
- Update your Windows system to the latest Service Pack. Windows XP SP2 for example should be upgraded to SP3 as it has known issues with some manufacturers anti-virus software.