Looks from your other post like you've already sussed it but as with the backslash you also have to escape quotes to include them in a string.

You want system to execute the following:
Code:
"%ProgramFiles%\Symantec\LiveUpdate\LUALL.exe"
So the code to do this is:
Code:
system("\"%ProgramFiles%\\Symantec\\LiveUpdate\\LUALL.exe\"");