When you run secure delete option to erase free disk space, Max PC Booster fills your hard drive with temporary files and overwrites them. Therefore, for a short period of time, there will be no space left on your hard drive and Windows warns you about this. You do not have to run Disk Cleanup, and can safely ignore the warning message, because when secure delete operation is completed, it will delete all temporary files it created and will return your full free space allocation.