[Robert Valerio]

WFP (Windows File Protection). These special files are stored mainly in two system folders, called System32 and Dllcache. In the System32 folder are stored all the files that are being used by the OS (with some exceptions), and they are untouchable if the system is running in normal way, because Windows is doing use of this files to work. In the Dllcache are stored the files used as back-up by the OS. These folders are located here (where is your XP installation drive letter):

X:\Windows\System32\
X:\Windows\System32\Dllcache\

In order to properly understand the function of these folders, you must understand what the WFP (Windows File Protection) system is and how it works. Well, WFP is a trick used by the OS in order to prevent possible attacks to the system from external sources, for example viruses, trojans or malicious programs. The system protects itself keeping healthy copies of the main files used by the OS in a hidden folder (Dllcache) and using them when a system file is overwritten by an unknown process, in this way, when you replace a file used by the OS and recognized as a system file, the OS automatically will replace it with its healthy copy, making impossible a virus propagation and stopping any possible attack.

http://www.amilcarguerra.com/~iceman/stuff/tutorials/replacing_sysfiles/03.php