This is perfectly normal and not an error. The Power Drawers installer and updater are careful to make sure all new files can be installed before installing any of them. Since Power Drawers is a shell enhancement, some program files may be in use by Windows when reinstalling or updating Power Drawers. 

Program files cannot be replaced while they are in use. By restarting Windows, the files become no longer "in use" and can be directly replaced. If the installer or updater tells you that it can't unload files, then the installer or updater should run automatically as soon as you restart Windows and then allow you to install normally. A quicker option instead of restarting Windows is to simply log off Windows and log back on again, since that will also free up "in use" files so they can be replaced.

You may have seen applications from other vendors that install updates to themselves and then require a restart. In such cases, some updated files are installed right away and then a restart allows other files to be installed. That means you briefly have incompatible files installed and that could cause a system crash. Since a restart is required anyway, we require a restart before installing anything so we can make sure your software version is stable. Our install/update system ensures that all new files are installed at the same time so you are ensured smooth, uninterrupted operation.