From: http://www.pcmag.com/article2/0,2817,2325762,00.asp

location outlook pst program files redirection virtual

For security, Vista forbids programs from writing to a variety of "sensitive" areas, among 
them the Windows folder and the Program Files folder. That includes all subfolders of Program 
Files, so this ban would be a hardship on older programs that store INI files or data files 
along with the program itself. To avoid disabling these programs, Windows Vista lies to them. 
When they try to write to a banned location, it says "Sure, no problem!" but actually saves 
the data in a holding area called the Virtual Store. And when the program tries to read back 
the data it wrote, Vista slips in the data from the Virtual Store. The old program works fine, 
and other programs aren't affected. A similar process handles programs that write to now-forbidden 
areas of the Registry.