I, my self, have had the need to use this, so I shall not waste time asking about your reasons. All I want you to know, is that you should use this, only if you really know what you are doing, because this might mess up winxp to a certain degree.
1. Navigate to:
2. Find ProgramFilesDir whose default should be C:\Program Files
3. Change this to whatever directory you wish. I changed it to D:\Program Files for example.