PF file extension is used by Windows to initially prefetch files that are about to be opened. Before a file is loaded, Windows has to check the corresponding directory if it has a PF file linked to the application. If it can find one, the PF file will be used to open the application directly. In cases where a PF file is not linked to the application, Windows will create the file and store it to the folder and load the application.
Most computers store PF files in C:\%windir%\prefetch directory. A prefetch (PF) file linked to an application and is updated once the application is accessed. Prefetch files are essential to Windows XP in loading applications faster. However, Prefetch folders can be disabled by accessing the registry.
A corrupted PF file or Prefetch folder can cause issues to the system. It is recommended to delete those prefetch files regularly. In addition, PF files for uninstalled applications can remain in the folder unless deleted. PF files can be manually deleted by opening the folder and deleting the files form there.
There are instances when worms and viruses are accessed through the PF files of a computer. Some anti-virus software applications are unable to scan the source file, but can still locate and open PF file linked to a virus or worm.
For more general information about files, file extensions and registry you can read one of the following articles:
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