Pagefile - When your system runs low on RAM because an application like Firefox is taking too much memory, Windows moves the least used "pages" of memory out to a hidden file named pagefile.sys in the root of one of your drives to free up more RAM for the applications you are actually using. What this actually means to you is that if you've had an application minimized for a while, and you are heavily using other applications, Windows is going to move some of the memory from the minimized application to the pagefile since it's not being accessed recently.
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http://lifehacker.com/5426041/understanding-the-windows-pagefile-and-why-you-shouldnt-disable-it
hiberfile.sys - Windows has two power management modes that you can choose from: one is Sleep Mode, which keeps the PC running in a low power state so you can almost instantly get back to what you were working on. The other is Hibernate mode, which completely writes the memory out to the hard drive, and then powers the PC down entirely, so you can even take the battery out, put it back in, start back up, and be right back where you were.
Hibernate mode uses the hiberfil.sys file to store the the current state (memory) of the PC, and since it’s managed by Windows, you can’t delete the file.
- http://www.howtogeek.com/howto/15140/what-is-hiberfil.sys-and-how-do-i-delete-it/