XP uses the C:/Program Files directory as the default base directory into which new programs are installed.
However, you can change the default installation drive and/or directory by using a Registry hack.
Run the Registry Editor and go to
Look for the value named ProgramFilesDir.
By default, the value will be C:/Program Files.
Edit the value to any valid drive or folder; XP will use that new location as the default installation directory for new programs.