Now many people do not like to install software in their c: drive.
It is a good practice to have most of your c drive empty for better performance of operating systems like windows xp.
So they install their programs in the other directories.
But some softwares are installed better in their automatic mode ,so in order to change the default directory do the following.
1.goto start> run>type regedit.
2.Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\currentversion
3.Here ere will be an entry called program files directory