change this registry value....please note to always make a back up of the registry just in case....a good free tool to use is ERUNT
before making any changes that way if you make a mistake or does not work as planned, you can always restore the registry back to where it was before made changes.
NT/W2K/XP's default directory for installing applications is the \Program Files folder (at least for English language version of NT/Windows 2000/XP). To change the default application installation folder:
Key: Software\Microsoft\Windows\Current Version
Value: C:\Program Files default
Please note that some programs ignore this registry setting.